Miniccpy is a small coupled-cluster (CC) package written fully in Python that implements several ground-state single-reference CC and equation-of-motion (EOM) CC methodologies in addition to some of the particle nonconserving EOMCC approaches of the singly and doubly electron attaching (EA) and ionizing (IP) varieties. Miniccpy serves as a freely available demonstration for how to program high-level CC/EOMCC methodologies in Python using Numpy, and it is a useful sandbox for exploring the development and implementation of new approaches. Miniccpy only deals with the correlated CC computation and uses interfaces to PySCF or GAMESS under the hood to obtain the starting Hartree-Fock solution and the associated one- and two-body molecular orbital integrals.

For simplicity, most of the routines in Miniccpy utilize the most general spinorbital form the CC/EOMCC equations, which does not assume that either the S_z or S² quantum numbers are conserved. Although the costs of the resulting calculations are substantially higher than those characterizing the commonly used spin-integrated form of the equations, which accomplishes a partial spin-adaptation by eliminating components of the equations that have different S_z values than the reference state, the spinorbital formulation produces the simplest and most general form of the CC/EOMCC equations that is applicable to a variety of many-body computations. This includes performing CC/EOMCC calculations using general spin-broken reference states (e.g., generalized Hartree-Fock, or GHF) as well as two-component relativistic approaches and nuclear structure calculations within the M-scheme.

• CCD
• CCSD
• CC3
• CCSDT
• CCSDTQ

• EOMCCSD
• EOM-CC3
• EOMCCSDT
• IP-EOMCCSD(2h-1p)
• IP-EOMCCSD(3h-2p)
• EA-EOMCCSD(2p-1h)
• EA-EOMCCSD(3p-2h)
• DIP-EOMCCSD(3h-1p)
• DIP-EOMCCSD(4h-2p)
• DEA-EOMCCSD(3p-1h)
• DEA-EOMCCSD(4p-2h)

Although Miniccpy is primarily a spinorbital CC code, it also offers a limited selection of computational options that exploit the RHF-based non-orthogonally spin-adapted formulation. This spin-free form of the CC/EOMCC equations allows for highly efficient computations, although it is only applicable to singlet states.

• CCD
• CCSD
• CC3
• CCSDT

• EOMCCSD
• EOMCCSDT

Karthik Gururangan
Doctoral student, Department of Chemistry, Michigan State University
e-mail: [email protected]

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