The EXP-T program package is designed for high-precision modeling of molecular electronic structure using the relativistic Fock space multireference coupled cluster method (FS-RCC). EXP-T is written from scratch in the C99 programming language and is currently focused on Unix-like systems.

**The public version of the EXP-T source code is available under LGPL on GitHub.**
Download source code (.tar.xz)
Download source code (GitHub)
Download manual (pdf)
## Electronic structure models implemented in EXP-T:

- single-point energy calculations with any point groups and (nearly) all
Hamiltonians, implemented in DIRAC (4c-DC, X2Cmmf, 2c-ECP, non-relativistic);
- ground state energy calculations: CCSD, CCSD(T), CCSDT-n (n=1,2,3), CCSDT models;
- FS-MRCC method (CCSD, CCSDT-1,2,3, CCSDT[3]) for excited states is implemented for the (0h1p), (1h0p), (1h1p), (0h2p), (2h0p), (0h3p) [2] Fock space sectors;
- (unrelaxed) analytic density matrix and properties for ground-state CCSD and CCSD(T);
- "dynamic" energy denominators shifts [4] as a solution of the intruder-state problem;
- Padé extrapolation to the zero-shift limit [5];
- intermediate Hamiltonians for incomplete main model spaces [7];
- finite-field transition moments calculations [6];
- quasidiabatization of SO-coupled states and SO extraction [4].

## Interfaces

To the moment the EXP-T package doesn’t include subroutines for solving (Dirac-) Hartree-Fock equations and subsequent four-index transformation, so molecular integrals have to be imported from third party electronic structure packages via interfaces. Currently EXP-T is interfaced with the

DIRAC program package, thus getting access to the wide variety of Hamiltonians and property operators implemented there.

## Features for high-performance calculations

- OpenMP parallelization (for shared-memory systems);
- parallel calculations on NVIDIA GPUs using the CUDA platform.

## Citation

We kindly ask you to acknowledge any use of the EXP-T program system that results in published material using the following citation:

A. V. Oleynichenko, A. Zaitsevskii, E. Eliav, Towards High Performance Relativistic Electronic Structure Modelling: The EXP-T Program Package.

*Commun. Comput. Inf. Sci.*, 1331, 375-386 (2020) doi:

10.1007/978-3-030-64616-5_33## References

[1] A. V. Oleynichenko, A. Zaitsevskii, E. Eliav

Towards High Performance Relativistic Electronic Structure Modelling: The EXP-T Program Package

*Commun. Comput. Inf. Sci.*, 1331, 375-386 (2020) doi:

10.1007/978-3-030-64616-5_33
[2] L. V. Skripnikov, A. V. Oleynichenko, A. V. Zaitsevskii, D. E. Maison, A. E. Barzakh

Relativistic Fock space coupled-cluster study of bismuth electronic structure to extract the Bi nuclear quadrupole moment

*Phys. Rev. C*, 104, 034316 (2021) doi:

10.1103/PhysRevC.104.034316
[3] A. V. Oleynichenko, A. Zaitsevskii, L. V. Skripnikov, E. Eliav

Relativistic Fock-Space Coupled Cluster Method for Many-Electron Systems: Non-Perturbative Account for Connected Triple Excitations

*Symmetry*, 12(7), 1101 (2020) doi:

10.3390/sym12071101
[4] A. Zaitsevskii, N. S. Mosyagin, A. V. Stolyarov, E. Eliav

Approximate relativistic coupled-cluster calculations on heavy alkali-metal diatomics: Application to the spin-orbit-coupled A

^{1}Σ

^{+} and b

^{3}Π states of RbCs and Cs

_{2}*Phys. Rev. A* 96, 022516 (2017) doi:

10.1103/PhysRevA.96.022516
[5] A. Zaitsevskii, E. Eliav

Padé extrapolated effective Hamiltonians in the Fock space relativistic coupled cluster method

*Int. J. Quantum Chem.* 118, e25772 (2018) doi:

10.1002/qua.25772
[6] A. V. Zaitsevskii, L. V. Skripnikov, A. V. Kudrin, A. V. Oleinichenko, E. Eliav, A. V. Stolyarov

Electronic Transition Dipole Moments in Relativistic Coupled-Cluster Theory: the Finite-Field Method

*Opt. Spectrosc.* 124, 451 (2018) doi:

10.1134/S0030400X18040215
[7] A. Zaitsevskii, N. S. Mosyagin, A. V. Oleynichenko, E. Eliav

Generalized relativistic small-core pseudopotentials accounting for quantum electrodynamic effects: construction and pilot applications

arXiv:2208.12296 [physics.atom-ph]