Analytic Gradients and Nonadiabatic Couplings for Device-Resident DMRG-QD-NEVPT2 Through Conical Intersections on a Consumer GPU
Abstract: Nonadiabatic dynamics through a conical intersection needs both static and dynamic correlation and, at every geometry, an excited-state gradient and interstate nonadiabatic coupling (NACME); analytic multireference derivatives at this level have been cluster-scale. We report device-resident \mbox{DMRG-QD-NEVPT2} with analytic derivatives, on B2PLYP double-hybrid Kohn--Sham orbitals: a density-matrix-renormalization-group reference supplies the static correlation, and quasi-degenerate NEVPT2 adds the dynamic correlation through a multi-state effective Hamiltonian that stays valid \emph{through} the intersection, where single-state perturbation theory fails. Each gradient and NACME is one reverse-mode transpose of a contraction graph that carries the DMRG sweep as a gauge-free, tape-free node and reduces to active-space RDM contractions with Cholesky factors. The whole construction runs on a consumer 8\,GB GPU. The DMRG reference is FCI-in-active-space to $10^{-15}$ and the single-precision leg is spectroscopically inert ($<10^{-6}$\,eV); the correction yields smooth adiabats and a non-vanishing, $1/ΔE$-divergent NACME through the twisted-ethene intersection---where adiabatic linear-response TDDFT returns zero---and restores the dynamic correlation the ionic $ππ^*$ states require. DMRG-NEVPT2 and quasi-degenerate NEVPT2 are each established; the contribution is the entire stack---multi-state through a conical intersection, with analytic gradients and couplings---made device-resident on commodity hardware. Multireference companion to our density-functional realization of the same engine.