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

Abstract

<jats:p>The Fock space relativistic coupled cluster method (FS-RCC) is one of the most promising tools of electronic structure modeling for atomic and molecular systems containing heavy nuclei. Until recently, capabilities of the FS-RCC method were severely restricted by the fact that only single and double excitations in the exponential parametrization of the wave operator were considered. We report the design and the first computer implementation of FS-RCC schemes with full and simplified non-perturbative account for triple excitations in the cluster operator. Numerical stability of the new computational scheme and thus its applicability to a wide variety of molecular electronic states is ensured using the dynamic shift technique combined with the extrapolation to zero-shift limit. Pilot applications to atomic (Tl, Pb) and molecular (TlH) systems reported in the paper indicate that the breakthrough in accuracy and predictive power of the electronic structure calculations for heavy-element compounds can be achieved. Moreover, the described approach can provide a firm basis for high-precision modeling of heavy molecular systems with several open shells, including actinide compounds.</jats:p>

Global and local approaches to population analysis: Bonding patterns in superheavy element compounds

“…Geometry optimizations and one-step projection analysis calculations were carried out using the DIRAC15 package [21]. An iterative projection analysis procedure [13] was implemented on the top of DIRAC.…”

Finite-Field Calculations of Transition Properties by the Fock Space Relativistic Coupled Cluster Method: Transitions between Different Fock Space Sectors

“…The construction of spinors and evaluation of one-and two-electron integrals was performed using the DIRAC19 program package [15,44], whereas all FS RCC calculations were carried out with the help of the EXP-T code [45][46][47]. The VIBROT code [48] was used to solve the vibrational problem and compute the excited-state lifetime of TlF.…”

Finite-Field Calculations of Transition Properties by the Fock Space Relativistic Coupled Cluster Method: Transitions between Different Fock Space Sectors

“…We bypassed this difficulty via choosing quite large sets of active particle spinors and using the adjustable denominator shift technique [43] to suppress the possible divergencies in the presence of intruder states (more specifically, we employed the simulated imaginary shifts [32]). To conserve the core separability of the original FS RCC scheme, shifts were never applied to the energy denominators in the equations for T (0h0p) amplitudes; a balanced treatment of excitation and deexcitation contributions to PΩP implied the use of non-shifted equations for T The construction of spinors and evaluation of one-and two-electron integrals was performed using the DIRAC19 program package [15,44] whereas all FS RCC calculations were carried out with the help of the EXP-T code [45][46][47]. The VIBROT code [48] was used to solve the vibrational problem and compute the excited-state lifetime of TlF.…”