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