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The EXP-T program system

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;
  • fast and powerful finite-order method for calculations of diagonal and off-diagonal matrix elements of one-electron property operators [8];
  • (unrelaxed) analytic density matrix and properties for ground-state CCSD and CCSD(T);
  • intermediate Hamiltonians for incomplete main model spaces [7];
  • "dynamic" energy denominators shifts [4] as a solution of the intruder-state problem;
  • Padé extrapolation to the zero-shift limit [5];
  • finite-field transition moments calculations [6];
  • quasidiabatization of SO-coupled states and SO extraction [4].


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.


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


[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 A1Σ+ and b3Π states of RbCs and Cs2
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
Int. J. Quantum Chem. 123(8), e27077 (2023) doi: 10.1002/qua.27077

[8] A. Zaitsevskii, A. V. Oleynichenko, E. Eliav
Theoretical molecular spectroscopy of actinide compounds: the ThO molecule
Mol. Phys. REHE 2022 Special Issue, e2236246 (2023) doi: 10.1080/00268976.2023.2236246