<scp>REG</scp> ‐ <scp>MP2</scp> : A New <scp>MP2</scp> Method Accelerated by the Regularization of the Fock Matrix and Energy Denominator Matrix

ABSTRACT In this work, we newly developed the REG–MP2 method, an extension of Møller–Plesset perturbation theory (MP2) accelerated via dual regularization (REG) of the Fock matrix and energy denominator matrix, based on canonical MP2 references for molecular systems. This technique retains the high efficiency of MP2 while introducing a regularization parameter to resolve numerical instability originating from ill‐conditioned energy denominator matrices during calculations. The accuracy of REG–MP2 was extensively validated across a range of open–shell and closed–shell molecules, yielding a mean absolute deviation (MAD) of only 0.32 kcal/mol in calculated energies relative to canonical MP2 results. Additionally, we developed three variants of REG–MP2: REGDF–MP2 (incorporating density fitting), REGFC–MP2 (with frozen–core approximation), and REGDF–FC–MP2 (combining both approximations). Using the cc–pVDZ basis set and 28 parallel cores, the average computational time per benchmarked complex for the five MP2 methods ranks as follows: REGDF–MP2 (0.24 s) &lt; REG–MP2 (0.28 s) &lt; REGFC–MP2 (0.79 s) &lt; REGDF–FC–MP2 (0.84 s) &lt; MP2 (11.02 s). Among these, REG–MP2 is recommended for its optimal balance of accuracy and efficiency. This work presents REG–MP2 and its variants as new efficient MP2–based approaches, suitable for quantum chemical calculations requiring balanced accuracy and efficiency with manageable computational costs.