Assessing the accuracy of simplified coupled cluster methods for electronic excited states in f0 actinide compounds

Abstract

We scrutinize the performance of different variants of equation of motion coupled cluster (EOM-CC) methods to predict electronic excitation energies and excited state potential energy surfaces in closedshell actinide species. We focus our analysis on various recently presented pair coupled cluster doubles (pCCD) models [J. Chem. Phys., 23, 234105 (2016) and J. Chem. Theory Comput. 15, 18-24 (2019)] and compare their performance to the conventional EOM-CCSD approach and to the completely renormalized EOM-CCSD with perturbative triples ansatz. Since the single-reference pCCD model allows us to efficiently describe static/nondynamic electron correlation, while dynamical electron correlation is accounted for a posteriori, the investigated pCCD-based methods represent a good compromise between accuracy and computational cost. Such a feature is particularly advantageous when modelling electronic structures of actinide-containing compounds with stretched bonds. Our work demonstrates that EOM-pCCD-based methods reliably predict electronic spectra of small actinide building blocks containing thorium, uranium, and protactinium atoms. Specifically, the standard errors in adiabatic and vertical excitation energies obtained by the conventional EOM-CCSD approach are reduced by a factor of 2 when employing the EOM-pCCD-LCCSD variant resulting in a mean error of 0.05 eV and a standard deviation of 0.25 eV.

Excited States from State-Specific Orbital-Optimized Pair Coupled Cluster

“…More precisely, the reference is described with a hybrid pCCD and linearized CC approach for the nonpair excitations, while all single and double excitations are included in the EOM excitation operator. Both methods have been used for computing excitation energies of singly and doubly excited states, ,,,, and very good performance was attained in these applications. However, excitation energies tend to be somewhat overestimated with respect to reference (FCI) values.…”

Density Matrices of Seniority-Zero Geminal Wavefunctions

“…Results building upon AP1roG/pCCD are quite promising. [159][160][161][162][163][164][165][166][167][168][169]…”

Orbital entanglement and correlation from pCCD-tailored coupled cluster wave functions

“…Some well known geminal-based methods are the antisymmetric product of strongly orthogonal geminals (APSG) [22,23], the antisymmetric product of interacting geminals (APIG) [24][25][26][27][28][29][30][31][32][33][34][35][36], and the antisymmetric product of 1-reference orbital geminals [37,38], also known as the pair-couled cluster doubles (pCCD) ansatz [39]. Numerical studies suggest that geminalbased approaches can accurately model systems where strong correlation is important, like the one-dimensional Hubbard model [38,40] or molecules with stretched bonds [36,39,[41][42][43][44][45][46][47], even those containing lanthanide [48] or actinide atoms [46,49,50]. Specifically, pCCD is a good approximation to the doubly occupied configuration interaction (DOCI) method [51], requires, however, only mean-field cost.…”

“…It remains uncertain whether an a posteriori correction is able to cure the deficiencies in electron correlation effects predicted by the pCCD approach. As post-pCCD method, we will choose the recently presented pCCD-LCC models [47] as they represents a promising alternative to conventional multireference electronic structure methods for both electronic ground [47] and excited states [50,63], where they statistically outperformed conventional CC methods. By scrutinizing the orbital entanglement and orbital-pair correlations predicted by pCCD-LCC, we will be able to complement previous numerical studies on the performance and reliability of the LCC corrections.…”