CCpy is a research-level Python package for performing non-relativistic electronic structure calculations for molecular systems using methods based on the ground-state coupled-cluster (CC) theory and its equation-of-motion (EOM) extension to electronic excited, attached, and ionized states. As a design philosophy, CCpy favors simplicity over efficiency, and this is reflected in the usage of computational routines that are transparent enough so that they can be easily used, modified, and extended, while still maintaining reasonable efficiency. To this end, CCpy employs a hybrid Python-Fortran programming approach made possible with the f2py package, which allows one to compile Fortran code into shared object libraries containing subroutines that are callable from Python and interoperable with Numpy arrays.
Important Note: The compliation of Fortran modules with f2py is not currently compatible with Python 3.12 due to
the migration away from numpy.distutils. In order to fix this, we are moving over to compilation using
meson. In the meantime, please use Python 3.11 when building CCpy.
CCpy specializes in applying the CC(P;Q) and externally corrected (ec) CC methodologies developed in the Piecuch group at Michigan State University. In CC(P;Q), the energetics obtained by solving the ground- or excited-state CC/EOMCC equations in one subspace of the many-electron Hilbert space, called the P space, are corrected for the missing many-electron correlation effects captured with the help of a complementary subspace called the Q space using the state-selective, non-iterative, and non-perturbative energy corrections based on the CC moment expansion formalism. Currently, CCpy offers implementations of several CC(P;Q) methods, the majority of which are aimed at converging the high-level CCSDT and EOMCCSDT energetics. These include the completely-renormalized (CR) methods such as the CR-CC(2,3) and CR-CC(2,4) triples and quadruples corrections to CCSD, the active-space CCSDt and CC(t;3) approaches, which are based on a user-defined selection of active orbitals, and the black-box selected configuration interaction (CI) driven and adaptive CC(P;Q) methodologies, which construct the P and Q spaces entering the CC(P;Q) computations using information extracted from selected CI wave functions or the adaptive CC(P;Q) moment expansions themselves, respectively. The ec-CC approaches on the other hand seek to converge the exact, full CI energetics directly by solving for the T1 and T2 clusters in the presence of the leading T3 and T4 clusters extracted from an external non-CC wave function. Current implementations of the ec-CC approaches in CCpy are designed to iterate T1 and T2 clusters in the presence of T3 and T4 obtained from CI wave functions of the selected CI or multireference CI types, and correct the resulting energetics for the missing many-electron correlations using the generalized moment expansions of the ec-CC equations.
- MP2
- MP3
- CCD
- CCSD
- CCSD(T)
- CR-CC(2,3)
- CC3
- CCSDt
- CC(t;3)
- CIPSI-driven CC(P;Q) aimed at converging CCSDT (see Ref. [1])
- Adaptive CC(P;Q) aimed at converging CCSDT (see Ref. [2])
- CCSDT
- CR-CC(2,4)
- CCSDTQ (available for closed shells only)
- ec-CC-II
- ec-CC-II3 (see Ref. [3])
- ec-CC_II3,4 (see Ref. [3])
- EOMCCSD
- CR-EOMCC(2,3) and its size-intensive δ-CR-EOMCC(2,3) extension
- EOM-CC3
- EOMCCSDt
- Excited-state CC(t;3)
- Adaptive CC(P;Q) aimed at converging EOMCCSDT
- EOMCCSDT
- SF-EOMCCSD
- SF-EOMCC(2,3)
- IP-EOMCCSD(2h-1p)
- IP-EOMCCSD(3h-2p)
- EA-EOMCCSD(2p-1h)
- EA-EOMCCSD(3p-2h)
- DEA-EOMCCSD(3p-1h)
- DEA-EOMCCSD(4p-2h)
- DIP-EOMCCSD(3h-1p)
- DIP-EOMCCSD(4h-2p)
Because CCpy is primarily used for CC method development work, we use interfaces to GAMESS and PySCF to obtain the mean-field (typically Hartree-Fock) reference state and associated one- and two-electron integrals in the molecular orbital basis prior to performing the correlated CC calculations. All implementations in CCpy are based on the spin-integrated spinorbital formulation and are compatible with RHF and ROHF references.
[1] K. Gururangan, J. E. Deustua, J. Shen, and P. Piecuch, J. Chem. Phys. 155, 174114 (2021)
(see https://doi.org/10.1063/5.0064400; cf. also https://doi.org/10.48550/arXiv.2107.10994)
[2] K. Gururangan and P. Piecuch, J. Chem. Phys. 159, 084108 (2023)
(see https://doi.org/10.1063/5.0162873; cf. also https://doi.org/10.48550/arXiv.2306.09638)
[3] I. Magoulas, K. Gururangan, P. Piecuch, J. E. Deustua, and J. Shen, J. Chem. Theory Comput. 17, 4006 (2021)
(see https://doi.org/10.1021/acs.jctc.1c00181; cf. also https://doi.org/10.48550/arXiv.2102.10143)
Installation instructions are provided in the CCpy documentation, which is created using sphinx.
Please see the docs directory for instructions on how to compile and view the documentation.
Karthik Gururangan
Doctoral student, Department of Chemistry, Michigan State University
e-mail: gururang@msu.edu
Dr. J. Emiliano Deustua
COO and Co-founder, Examol
Professor Piotr Piecuch
University Distinguished Professor and Michigan State University Foundation Professor, Department of Chemistry, Michigan State University
Adjunct Professor, Department of Physics and Astronomy, Michigan State University
CCpy is an open-source code under the GPLv3 license developed and maintained by the Piecuch Group at Michigan State University. In addition to using GitHub's Issues feature, feel free to send an e-mail to gururang@msu.edu if you have any questions about using CCpy or are seeking additional information about its functionality.