Towards Multireference Equivalents of the HEAT Thermochemical Protocol

This study systematically evaluates the performance of internally contracted multireference coupled cluster (icMRCC) wave functions constructed using a full-valence complete active space reference as an alternative electronic structure method within the high-accuracy extrapolated ab initio thermochemistry (HEAT) protocol, thereby assessing the accuracy of icMRCC and exploring its potential for highly accurate thermochemical predictions. By substituting single-reference wavefunctions with multireference (MR) alternatives, we aim to capture complex electron correlation effects, particularly in systems with strong static correlations. Using a benchmark dataset of 22 small first-row compounds, we compare the accuracy of different icMRCCSD(T) methodologies with both single-reference their counterparts and experimental data. Our results align with prior findings, confirming that the intrinsic error of the icMRCCSD(T){4}_F method remains well below the chemical accuracy threshold (∼4 kJ mol^-1) for thermochemical properties, particularly for atomization energies of molecules with up to 18 correlated electrons. The results underscore the potential of the methods for creating a multireference framework as a high-precision tool for thermochemical applications.