Full configuration interaction quantum Monte Carlo benchmark and multireference coupled cluster studies of tetramethyleneethane diradical

TL;DR

This study provides a high-accuracy benchmark for tetramethyleneethane's potential energy surface using FCIQMC, and evaluates the performance of multireference coupled cluster and DMRG-tailored CCSD methods against this benchmark.

Contribution

It offers the first FCIQMC benchmark for tetramethyleneethane and assesses the accuracy of advanced multireference coupled cluster and DMRG methods.

Findings

FCIQMC results agree with previous calculations and experiments.

MR MkCCSDT captures energy gaps but not PES shape.

DMRG(24,25) fails to include dynamic correlation, affecting PES shape.

Abstract

We have performed a FCI-quality benchmark calculation for the tetramethyleneethane molecule in cc-pVTZ basis set employing a subset of CASPT2(6,6) natural orbitals for the FCIQMC calculation. The results are in an excellent agreement with the previous large scale diffusion Monte Carlo calculations by Pozun et al. and available experimental results. Our computations verified that there is a maximum on PES of the ground singlet state ($^1\text{A}$) $45^{\circ}$ torsional angle and the corresponding vertical singlet-triplet energy gap is $0.01$ eV. We have employed this benchmark for assessment of the accuracy of MkCCSDT and DMRG-tailored CCSD (TCCSD) methods. MR MkCCSDT with CAS(2,2) model space, though giving good values for the singlet-triplet energy gap, is not able to properly describe the shape of the multireference singlet PES. Similarly, DMRG(24,25) is not able to correctly capture the shape of the singlet surface, due to the missing dynamic correlation. On the other hand, the DMRG-tailored CCSD method describes the shape of the ground singlet state with an excellent accuracy, but for the correct ordering requires computation of the zero-spin-projection component of the triplet state ($^3\text{B}_1$).