Coskun, M.Koehn, A.Ertuerk, M.2026-02-032026-02-0320250192-86511096-987Xhttps://doi.org/10.1002/jcc.70286https://hdl.handle.net/20.500.12428/35006This 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 (similar to 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.eninfo:eu-repo/semantics/closedAccessatomization energycomposite schemesmultireference coupled cluster methodreaction enthalpythermochemistryArticle463110.1002/jcc.70286Q2WOS:0016324035000112-s2.0-105023231824Q1