DMRGSCF (PySCF)

In this section we explain how to use block2 (and optionally StackBlock) and pyscf for DMRGSCF (CASSCF with DMRG as the active space solver).

Preparation

pyscf can be installed using pip install pyscf. One also needs to install the pyscf extension called dmrgscf, which can be obtained from https://github.com/pyscf/dmrgscf. If it is installed using pip, one also needs to create a file named settings.py under the dmrgscf folder, as follows:

$ pip install git+https://github.com/pyscf/dmrgscf
$ PYSCFHOME=$(pip show pyscf-dmrgscf | grep 'Location' | tr ' ' '\n' | tail -n 1)
$ wget https://raw.githubusercontent.com/pyscf/dmrgscf/master/pyscf/dmrgscf/settings.py.example
$ mv settings.py.example ${PYSCFHOME}/pyscf/dmrgscf/settings.py
$ chmod +x ${PYSCFHOME}/pyscf/dmrgscf/nevpt_mpi.py

Here we also assume that you have installed block2 either using pip or manually.

DMRGSCF (serial)

The following is an example python script for DMRGSCF using block2 running in a single node without MPI parallelism:

from pyscf import gto, scf, lib, dmrgscf
import os

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='C 0 0 0; C 0 0 1.2425', basis='ccpvdz',
    symmetry='d2h', verbose=4, max_memory=10000) # mem in MB
mf = scf.RHF(mol)
mf.kernel()

from pyscf.mcscf import avas
nactorb, nactelec, coeff = avas.avas(mf, ["C 2p", "C 3p", "C 2s", "C 3s"])
print('CAS = ', nactorb, nactelec)

mc = dmrgscf.DMRGSCF(mf, nactorb, nactelec, maxM=1000, tol=1E-10)
mc.fcisolver.runtimeDir = lib.param.TMPDIR
mc.fcisolver.scratchDirectory = lib.param.TMPDIR
mc.fcisolver.threads = int(os.environ.get("OMP_NUM_THREADS", 4))
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB

mc.canonicalization = True
mc.natorb = True
mc.kernel(coeff)

Note

Alternatively, to use StackBlock instead of block2 as the DMRG solver, one can change the line involving dmrgscf.settings.BLOCKEXE to:

dmrgscf.settings.BLOCKEXE = os.popen("which block.spin_adapted").read().strip()

Please see MPS Import/Export for the instruction for the installation of StackBlock.

Note

It is important to set a suitable mc.fcisolver.threads if you have multiple CPU cores in the node, to get high efficiency.

This will generate the following output:

$ grep 'CASSCF energy' cas1.out
CASSCF energy = -75.6231442712648

DMRGSCF (distributed parallel)

The following example is DMRGSCF in hybrid MPI (distributed) and openMP (shared memory) parallelism. For example, we can use 7 MPI processors and each processor uses 4 threads (so in total the calculation will be done with 28 CPU cores):

from pyscf import gto, scf, lib, dmrgscf
import os

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = 'mpirun -n 7 --bind-to none'

mol = gto.M(atom='C 0 0 0; C 0 0 1.2425', basis='ccpvdz',
    symmetry='d2h', verbose=4, max_memory=10000) # mem in MB
mf = scf.RHF(mol)
mf.kernel()

from pyscf.mcscf import avas
nactorb, nactelec, coeff = avas.avas(mf, ["C 2p", "C 3p", "C 2s", "C 3s"])
print('CAS = ', nactorb, nactelec)

mc = dmrgscf.DMRGSCF(mf, nactorb, nactelec, maxM=1000, tol=1E-10)
mc.fcisolver.runtimeDir = lib.param.TMPDIR
mc.fcisolver.scratchDirectory = lib.param.TMPDIR
mc.fcisolver.threads = 4
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB

mc.canonicalization = True
mc.natorb = True
mc.kernel(coeff)

Note

To use MPI with block2, the block2 must be either (a) installed using pip install block2-mpi or (b) manually built with -DMPI=ON. Note that the block2 installed using pip install block2 cannot be used together with mpirun if there are more than one processors (if this happens, it will generate wrong results and undefined behavior).

If you have already pip install block2, you must first pip uninstall block2 then pip install block2-mpi.

Note

If you do not have the --bind-to option in the mpirun command, sometimes every processor will only be able to use one thread (even if you set a larger number in the script), which will decrease the CPU usage and efficiency.

This will generate the following output:

$ grep 'CASSCF energy' cas2.out
CASSCF energy = -75.6231442712753

CASSCF Reference

For this small (8, 8) active space, we can also compare the above DMRG results with the CASSCF result:

from pyscf import gto, scf, lib, mcscf
import os

mol = gto.M(atom='C 0 0 0; C 0 0 1.2425', basis='ccpvdz',
    symmetry='d2h', verbose=4, max_memory=10000) # mem in MB
mf = scf.RHF(mol)
mf.kernel()

from pyscf.mcscf import avas
nactorb, nactelec, coeff = avas.avas(mf, ["C 2p", "C 3p", "C 2s", "C 3s"])
print('CAS = ', nactorb, nactelec)

mc = mcscf.CASSCF(mf, nactorb, nactelec)
mc.fcisolver.conv_tol = 1E-10
mc.canonicalization = True
mc.natorb = True
mc.kernel(coeff)

This will generate the following output:

$ grep 'CASSCF energy' cas3.out
CASSCF energy = -75.6231442712446

State-Average with Different Spins

The following is an example python script for state-averaged DMRGSCF with singlet and triplet:

from pyscf import gto, scf, lib, dmrgscf, mcscf
import os

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='C 0 0 0; C 0 0 1.2425', basis='ccpvdz',
    symmetry='d2h', verbose=4, max_memory=10000) # mem in MB
mf = scf.RHF(mol)
mf.kernel()

from pyscf.mcscf import avas
nactorb, nactelec, coeff = avas.avas(mf, ["C 2p", "C 3p", "C 2s", "C 3s"])
print('CAS = ', nactorb, nactelec)

lib.param.TMPDIR = os.path.abspath(lib.param.TMPDIR)

solvers = [dmrgscf.DMRGCI(mol, maxM=1000, tol=1E-10) for _ in range(2)]
weights = [1.0 / len(solvers)] * len(solvers)

solvers[0].spin = 0
solvers[1].spin = 2

for i, mcf in enumerate(solvers):
    mcf.runtimeDir = lib.param.TMPDIR + "/%d" % i
    mcf.scratchDirectory = lib.param.TMPDIR + "/%d" % i
    mcf.threads = 8
    mcf.memory = int(mol.max_memory / 1000) # mem in GB

mc = mcscf.CASSCF(mf, nactorb, nactelec)
mcscf.state_average_mix_(mc, solvers, weights)

mc.canonicalization = True
mc.natorb = True
mc.kernel(coeff)

Note

The mc parameter in the function state_average_mix_ must be a CASSCF object. It cannot be a DMRGSCF object (will produce a runtime error).

This will generate the following output:

$ grep 'State ' cas4.out
State 0 weight 0.5  E = -75.6175232350073 S^2 = 0.0000000
State 1 weight 0.5  E = -75.298522666384  S^2 = 2.0000000

Unrestricted DMRGSCF

One can also perform Unrestricted CASSCF (UCASSCF) with block2 using a UHF reference. Currently this is not directly supported by the pyscf/dmrgscf package, but here we can add some small modifications. The following is an example:

from pyscf import gto, scf, lib, dmrgscf, mcscf, fci
import os

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='C 0 0 0; C 0 0 1.2425', basis='ccpvdz',
    symmetry=False, verbose=4, max_memory=10000) # mem in MB
mf = scf.UHF(mol)
mf.kernel()

def write_uhf_fcidump(DMRGCI, h1e, g2e, n_sites, nelec, ecore=0, tol=1E-15):

    import numpy as np
    from pyscf import ao2mo
    from subprocess import check_call
    from block2 import FCIDUMP, VectorUInt8

    if isinstance(nelec, (int, np.integer)):
        na = nelec // 2 + nelec % 2
        nb = nelec - na
    else:
        na, nb = nelec

    assert isinstance(h1e, tuple) and len(h1e) == 2
    assert isinstance(g2e, tuple) and len(g2e) == 3

    mh1e_a = h1e[0][np.tril_indices(n_sites)]
    mh1e_b = h1e[1][np.tril_indices(n_sites)]
    mh1e_a[np.abs(mh1e_a) < tol] = 0.0
    mh1e_b[np.abs(mh1e_b) < tol] = 0.0

    g2e_aa = ao2mo.restore(8, g2e[0], n_sites)
    g2e_bb = ao2mo.restore(8, g2e[2], n_sites)
    g2e_ab = ao2mo.restore(4, g2e[1], n_sites)
    g2e_aa[np.abs(g2e_aa) < tol] = 0.0
    g2e_bb[np.abs(g2e_bb) < tol] = 0.0
    g2e_ab[np.abs(g2e_ab) < tol] = 0.0

    mh1e = (mh1e_a, mh1e_b)
    mg2e = (g2e_aa, g2e_bb, g2e_ab)

    cmd = ' '.join((DMRGCI.mpiprefix, "mkdir -p", DMRGCI.scratchDirectory))
    check_call(cmd, shell=True)
    if not os.path.exists(DMRGCI.runtimeDir):
        os.makedirs(DMRGCI.runtimeDir)

    fd = FCIDUMP()
    fd.initialize_sz(n_sites, na + nb, na - nb, 1, ecore, mh1e, mg2e)
    fd.orb_sym = VectorUInt8([1] * n_sites)
    integral_file = os.path.join(DMRGCI.runtimeDir, DMRGCI.integralFile)
    fd.write(integral_file)
    DMRGCI.groupname = None
    DMRGCI.nonspinAdapted = True
    return integral_file

def make_rdm12s(DMRGCI, state, norb, nelec, **kwargs):

    import numpy as np

    if isinstance(nelec, (int, np.integer)):
        na = nelec // 2 + nelec % 2
        nb = nelec - na
    else:
        na, nb = nelec

    file2pdm = "2pdm-%d-%d.npy" % (state, state) if DMRGCI.nroots > 1 else "2pdm.npy"
    dm2 = np.load(os.path.join(DMRGCI.scratchDirectory, "node0", file2pdm))
    dm2 = dm2.transpose(0, 1, 4, 2, 3)
    dm1a = np.einsum('ikjj->ki', dm2[0]) / (na - 1)
    dm1b = np.einsum('ikjj->ki', dm2[2]) / (nb - 1)

    return (dm1a, dm1b), dm2

dmrgscf.dmrgci.writeIntegralFile = write_uhf_fcidump
dmrgscf.DMRGCI.make_rdm12s = make_rdm12s

mc = mcscf.UCASSCF(mf, 8, 8)
mc.fcisolver = dmrgscf.DMRGCI(mol, maxM=1000, tol=1E-7)
mc.fcisolver.runtimeDir = lib.param.TMPDIR
mc.fcisolver.scratchDirectory = lib.param.TMPDIR
mc.fcisolver.threads = int(os.environ["OMP_NUM_THREADS"])
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB

mc.canonicalization = True
mc.natorb = True
mc.kernel()

Note

In the above example, mf is the UHF object and mc is the UCASSCF object. It is important to ensure that both of them are with unrestricted orbitals. Otherwise the calculation may be done with only restricted orbitals. DMRGSCF wrapper cannot be used for this example.

Note

Due to limitations in pyscf/UCASCI, currently the point group symmetry is not supported in UCASSCF/UCASCI with DMRG solver. pyscf/avas does not support creating active space with unrestricted orbtials so here we did not use avas. The above example will not work with StackBlock (the compatibility with StackBlock will be considered in future).

This will generate the following output:

$ grep 'UCASSCF energy' cas5.out
UCASSCF energy = -75.6231442541606

UCASSCF Reference

We compare the above DMRG results with the UCASSCF result using the FCI solver:

mc = mcscf.UCASSCF(mf, 8, 8)
mc.fcisolver.conv_tol = 1E-10
mc.canonicalization = True
mc.natorb = True
mc.kernel(coeff)

This will generate the following output:

$ grep 'UCASSCF energy' cas6.out
UCASSCF energy = -75.6231442706386

DMRGSCF Nuclear Gradients and Geometry Optimization

The following is an example python script for computing DMRGSCF nuclear gradients and geometry optimization using block2:

from pyscf import gto, scf, lib, dmrgscf
import os

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='C 0 0 0; C 0 0 1.2425', basis='ccpvdz',
    symmetry='d2h', verbose=4, max_memory=10000) # mem in MB
mf = scf.RHF(mol)
mf.kernel()

from pyscf.mcscf import avas
nactorb, nactelec, coeff = avas.avas(mf, ["C 2p", "C 3p", "C 2s", "C 3s"])
print('CAS = ', nactorb, nactelec)

mc = mcscf.CASSCF(mf, nactorb, nactelec)
mc.fcisolver = dmrgscf.DMRGCI(mol, maxM=1000, tol=1E-10)
mc.fcisolver.runtimeDir = lib.param.TMPDIR
mc.fcisolver.scratchDirectory = lib.param.TMPDIR
mc.fcisolver.threads = int(os.environ.get("OMP_NUM_THREADS", 4))
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB

mc.canonicalization = True
mc.natorb = True
mc.kernel(coeff)

grad = mc.nuc_grad_method().kernel()

mol_eq = mc.nuc_grad_method().optimizer(solver='geomeTRIC').kernel()
print(mol_eq.atom_coords())

This will generate the following output (the nuclear gradient at the initial geometry and the optimized geometry):

$ grep -A 4 'SymAdaptedCASSCF gradients' cas7.out
--------------- SymAdaptedCASSCF gradients ---------------
        x                y                z
0 C     0.0000000000     0.0000000000     0.0388202961
1 C     0.0000000000     0.0000000000    -0.0388202961
----------------------------------------------
$ tail -n 3 cas7.out
cycle 3: E = -75.6240204052  dE = -5.51573e-07  norm(grad) = 9.37108e-05
[[ 0.          0.         -1.19709701]
[ 0.          0.          1.19709701]]

Note

Currently, gradients for UCASSCF is not supported in pyscf. The geometry optimization part requires an additional module called geomeTRIC, which can be installed via pip install geometric.

DMRG-SC-NEVPT2

The following is an example python script for a DMRG-SC-NEVPT2 calculation (with explicit 4pdm) using block2:

from pyscf import gto, scf, mcscf, mrpt, dmrgscf, lib
import os

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='O 0 0 0; O 0 0 1.207', basis='cc-pvdz', spin=2, verbose=4)
mf = scf.RHF(mol).run(conv_tol=1E-20)

mc = mcscf.CASSCF(mf, 6, 8)

mc.fcisolver = dmrgscf.DMRGCI(mol, maxM=500, tol=1E-10)
mc.fcisolver.runtimeDir = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.scratchDirectory = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.threads = 8
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB

mc.fcisolver.conv_tol = 1e-14
mc.canonicalization = True
mc.natorb = True
mc.run()

sc = mrpt.NEVPT(mc).run()

The alternative faster compress_approx approach using MPS compression is also supported:

from pyscf import gto, scf, mcscf, mrpt, dmrgscf, lib
import os

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.BLOCKEXE_COMPRESS_NEVPT = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='O 0 0 0; O 0 0 1.207', basis='cc-pvdz', spin=2, verbose=4)
mf = scf.RHF(mol).run(conv_tol=1E-20)

mc = mcscf.CASSCF(mf, 6, 8)

mc.fcisolver = dmrgscf.DMRGCI(mol, maxM=500, tol=1E-10)
mc.fcisolver.runtimeDir = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.scratchDirectory = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.threads = 8
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB

mc.fcisolver.conv_tol = 1e-14
mc.canonicalization = True
mc.natorb = True
mc.run()

sc = mrpt.NEVPT(mc).compress_approx(maxM=200).run()

This will generate the following output (for compress_approx approach):

$ grep 'CASSCF energy' sc-nevpt2.out
CASSCF energy = -149.708657771219
$ grep 'Nevpt2 Energy' sc-nevpt2.out
Nevpt2 Energy = -0.249182302692906

So the total NEVPT2 energy using the compress_approx approach is -149.708657771219 + -0.249182302692906 = -149.9578400739119.

Note

The first “4pdm” approach is not supported by StackBlock, but it is supported in the old Block code. The second “compression” approach is supported by StackBlock. Block2 supports both approaches.

When using the second approach, it will generate a warning saying that WARN: DMRG executable file for nevptsolver is the same to the executable file for DMRG solver. If they are both compiled by MPI compilers, they may cause error or random results in DMRG-NEVPT calculation.. Please ignore this warning for block2. For block2, it is okay to set BLOCKEXE and BLOCKEXE_COMPRESS_NEVPT to the same file. BLOCKEXE_COMPRESS_NEVPT can be compiled with or without MPI. So only a single version of block2main is required. If you want to use MPI, please set both BLOCKEXE and BLOCKEXE_COMPRESS_NEVPT to the same block2main and compile block2 with MPI, or use pip install block2-mpi, and then set an appropriate MPIPREFIX.

The second “compression” approach requires the mpi4py python package. Make sure import mpi4py works in python before trying this example. Also, make sure that the file ${PYSCFHOME}/pyscf/dmrgscf/nevpt_mpi.py has the execute permission. You can do chmod +x ${PYSCFHOME}/pyscf/dmrgscf/nevpt_mpi.py to fix the permission.

Note that for the second “compression” approach, if you need to add any extra keywords for the DMRG solver, such as singlet_embedding, you need to add it using mc.fcisolver.block_extra_keyword instead of mc.fcisolver.extraline.

DMRG-SC-NEVPT2 (Multi-State)

The following is an example input file for state-averaged DMRGSCF for three states, and then the SC-NEVPT2 treatment of each of the three states.

import numpy as np
from pyscf import gto, scf, mcscf, mrpt, dmrgscf, lib
import os

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.BLOCKEXE_COMPRESS_NEVPT = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='O 0 0 0; O 0 0 1.207', basis='cc-pvdz', spin=2, verbose=4)
mf = scf.RHF(mol).run(conv_tol=1E-20)

# state average casscf
mc = mcscf.CASSCF(mf, 6, 8)
mc.fcisolver = dmrgscf.DMRGCI(mol, maxM=500, tol=1E-10)
mc.fcisolver.runtimeDir = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.scratchDirectory = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.threads = 8
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB
mc.fcisolver.conv_tol = 1e-14
mc.fcisolver.nroots = 3
mc = mcscf.state_average_(mc, [1.0 / 3] * 3)
mc.kernel()
mf.mo_coeff = mc.mo_coeff

# need an extra casci before calling mrpt
mc = mcscf.CASCI(mf, 6, 8)
mc.fcisolver = dmrgscf.DMRGCI(mol, maxM=500, tol=1E-10)
mc.fcisolver.runtimeDir = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.scratchDirectory = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.threads = 8
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB
mc.fcisolver.conv_tol = 1e-14
mc.fcisolver.nroots = 3
mc.natorb = True
mc.kernel()

# canonicalization for each state
ms = [None] * mc.fcisolver.nroots
cs = [None] * mc.fcisolver.nroots
es = [None] * mc.fcisolver.nroots
for ir in range(mc.fcisolver.nroots):
    ms[ir], cs[ir], es[ir] = mc.canonicalize(mc.mo_coeff, ci=mc.ci[ir], cas_natorb=False)

refs = [-149.956650684550, -149.725338427894, -149.725338427894]

# mrpt
for ir in range(mc.fcisolver.nroots):
    mc.mo_coeff, mc.ci, mc.mo_energy = ms[ir], cs, es[ir]
    mr = mrpt.nevpt2.NEVPT(mc).set(canonicalized=True).compress_approx(maxM=200).run(root=ir)
    print('root =', ir, 'E =', mc.e_tot[ir] + mr.e_corr, 'diff =', mc.e_tot[ir] + mr.e_corr - refs[ir])

This will generate the following output:

$ grep 'diff' multi.out
root = 0 E = -149.95664910937998 diff = 1.5751700175314909e-06
root = 1 E = -149.72529848179465 diff = 3.994609934920845e-05
root = 2 E = -149.7252985999243 diff = 3.9827969715133804e-05

Note

The above script should generate the same result if the explicit 4PDM approach is used, by removing .compress_approx(maxM=200).

Changing mc.fcisolver to the default FCI active space solver should also generate the same result (note that .compress_approx(maxM=200) is not supported by the FCI active space solver).

When the FCI active space solver is used, explicit canonicalization is also optional, namely, one can also remove .set(canonicalized=True) and mc.mo_coeff, mc.ci, mc.mo_energy = ms[ir], cs, es[ir] and the result will still be the same.

DMRG-IC-NEVPT2

The following is an example python script for SC-NEVPT2 / IC-NEVPT2 with equations derived on the fly (using the FCI solver):

import numpy
from pyscf import gto, scf, mcscf

mol = gto.M(atom='O 0 0 0; O 0 0 1.207', basis='cc-pvdz', spin=2, verbose=4)
mf = scf.RHF(mol).run(conv_tol=1E-20)

mc = mcscf.CASSCF(mf, 6, 8)
mc.fcisolver.conv_tol = 1e-14
mc.conv_tol = 1e-11
mc.canonicalization = True
mc.run()

from pyblock2.icmr.scnevpt2 import WickSCNEVPT2
wsc = WickSCNEVPT2(mc).run()

from pyblock2.icmr.icnevpt2_full import WickICNEVPT2
wic = WickICNEVPT2(mc).run()

This will generate the following output:

$ grep 'E(WickSCNEVPT2)' nevpt2.out
E(WickSCNEVPT2) = -149.9578403403482  E_corr_pt = -0.2491825691128931
$ grep 'E(WickICNEVPT2)' nevpt2.out
E(WickICNEVPT2) = -149.9601376470851  E_corr_pt = -0.2514798758497859

The above example can also run with the block2 DMRG solver:

import numpy
from pyscf import gto, scf, mcscf, dmrgscf, lib
import os

if not os.path.exists(lib.param.TMPDIR):
    os.mkdir(lib.param.TMPDIR)

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='O 0 0 0; O 0 0 1.207', basis='cc-pvdz', spin=2, verbose=4)
mf = scf.RHF(mol).run(conv_tol=1E-20)

mc = mcscf.CASSCF(mf, 6, 8)

mc.fcisolver = dmrgscf.DMRGCI(mol, maxM=500, tol=1E-14)
mc.fcisolver.runtimeDir = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.scratchDirectory = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.threads = 28
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB

# set very tight thresholds for small system
mc.fcisolver.scheduleSweeps = [0, 4, 8, 12, 16]
mc.fcisolver.scheduleMaxMs = [250, 500, 500, 500, 500]
mc.fcisolver.scheduleTols = [1e-08, 1e-10, 1e-12, 1e-12, 1e-12]
mc.fcisolver.scheduleNoises = [0.0001, 0.0001, 5e-05, 5e-05, 0.0]
mc.fcisolver.maxIter = 30
mc.fcisolver.twodot_to_onedot = 20
mc.fcisolver.block_extra_keyword = ['singlet_embedding', 'full_fci_space', 'fp_cps_cutoff 0', 'cutoff 0']

mc.fcisolver.conv_tol = 1e-14
mc.conv_tol = 1e-11
mc.canonicalization = True
mc.run()

from pyblock2.icmr.scnevpt2 import WickSCNEVPT2
wsc = WickSCNEVPT2(mc).run()

from pyblock2.icmr.icnevpt2_full import WickICNEVPT2
wic = WickICNEVPT2(mc).run()

This will generate the following output:

$ grep 'E(WickSCNEVPT2)' dmrg-nevpt2.out
E(WickSCNEVPT2) = -149.9578400627551  E_corr_pt = -0.2491822915198339
$ grep 'E(WickICNEVPT2)' dmrg-nevpt2.out
E(WickICNEVPT2) = -149.9601376425396  E_corr_pt = -0.2514798713043632

DMRG-FIC-MRCISD

The following is an example python script for fully internally contracted MRCISD with equations derived on the fly (using the FCI solver):

# need first import numpy (before pyblock2)
# otherwise the numpy multi-threading may not work
import numpy

from pyscf import gto, scf, mcscf
from pyblock2.icmr.icmrcisd_full import WickICMRCISD

mol = gto.M(atom='O 0 0 0; O 0 0 1.207', basis='6-31g', spin=2, verbose=4)
mf = scf.RHF(mol).run(conv_tol=1E-20)

mc = mcscf.CASSCF(mf, 6, 8)
mc.fcisolver.conv_tol = 1e-14
mc.conv_tol = 1e-11
mc.run()

mol.verbose = 5
wsc = WickICMRCISD(mc).run()

This will generate the following output:

$ grep 'CASSCF energy' mrci.out
CASSCF energy = -149.636563280267
$ grep 'WickICMRCISD' mrci.out
E(WickICMRCISD)   = -149.7792742741091  E_corr_ci = -0.1427109938418027
E(WickICMRCISD+Q) = -149.7858102349944  E_corr_ci = -0.1492469547270254

Similarly, we can do DMRG-FIC-MRCISD:

# need first import numpy (before pyblock2)
# otherwise the numpy multi-threading may not work
import numpy

from pyscf import gto, scf, mcscf, dmrgscf, lib
from pyblock2.icmr.icmrcisd_full import WickICMRCISD
import os

if not os.path.exists(lib.param.TMPDIR):
    os.mkdir(lib.param.TMPDIR)

dmrgscf.settings.BLOCKEXE = os.popen("which block2main").read().strip()
dmrgscf.settings.MPIPREFIX = ''

mol = gto.M(atom='O 0 0 0; O 0 0 1.207', basis='6-31g', spin=2, verbose=4)
mf = scf.RHF(mol).run(conv_tol=1E-20)

mc = mcscf.CASSCF(mf, 6, 8)

mc.fcisolver = dmrgscf.DMRGCI(mol, maxM=500, tol=1E-14)
mc.fcisolver.runtimeDir = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.scratchDirectory = os.path.abspath(lib.param.TMPDIR)
mc.fcisolver.threads = 28
mc.fcisolver.memory = int(mol.max_memory / 1000) # mem in GB

# set very tight thresholds for small system
mc.fcisolver.scheduleSweeps = [0, 4, 8, 12, 16]
mc.fcisolver.scheduleMaxMs = [250, 500, 500, 500, 500]
mc.fcisolver.scheduleTols = [1e-08, 1e-10, 1e-12, 1e-12, 1e-12]
mc.fcisolver.scheduleNoises = [0.0001, 0.0001, 5e-05, 5e-05, 0.0]
mc.fcisolver.maxIter = 30
mc.fcisolver.twodot_to_onedot = 20
mc.fcisolver.block_extra_keyword = ['singlet_embedding', 'full_fci_space', 'fp_cps_cutoff 0', 'cutoff 0']

mc.fcisolver.conv_tol = 1e-14
mc.conv_tol = 1e-11
mc.run()

mol.verbose = 5
wsc = WickICMRCISD(mc).run()

This will generate the following output:

$ grep 'CASSCF energy' dmrg-mrci.out
CASSCF energy = -149.636563280264
$ grep 'WickICMRCISD' dmrg-mrci.out
E(WickICMRCISD)   = -149.7792742857885  E_corr_ci = -0.1427110055241769
E(WickICMRCISD+Q) = -149.785810250064  E_corr_ci = -0.1492469697996863

Note

The current FIC-MRCI / DMRG-FIC-MRCI implementation requires the explicit construction of the MRCI Hamiltonian, which is not practical for production runs.