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An analytical gradient theory for single-state N-electron valence stateperturbation theory (NEVPT2), using both strongly contracted (SC) and partiallycontracted (PC) internal contraction schemes, is developed. We demonstrate theutility of the developed algorithm in the optimization of the single-statemolecular geometry of acrolein, benzyne, benzene, the retinal chromophore PSB3,the GFP chromophore pHBI, and porphine, with the cc-pVTZ basis sets. TheSC-NEVPT2 analytical gradients exhibit numerical instability due to the lack ofinvariance with respect to the rotations among the inactive orbitals. On theother hand, PC-NEVPT2 gives molecular geometries comparable to CASPT2 in anytested cases. We discuss possible future developments that will make the NEVPT2gradient algorithm a powerful tool for optimizing the molecular geometries andconducting molecular dynamics simulations of correlated systems.

Analytical Gradient Theory for Strongly Contracted (SC-) and Partially Contracted (PC-) N-Electron Valence State Perturbation Theory (NEVPT2)