The spin-unrestricted Hartree-Fock (UHF) method effectively accounts for static electron correlation, albeit at the expense of broken spin symmetry in the wavefunction. Such broken-symmetry UHF solutions suffer from spin contamination, which degrades the accuracy of molecular properties and excited-state calculations. Spin symmetry can be fully or partially restored by various projection approaches; including Löwdin's projection method, an approximate spin-projection scheme based on the Heisenberg Hamiltonian, and the projected generator coordinate method. In this paper, the linear-scaling divide-and-conquer (DC) UHF method is combined with these spin-projection approaches to treat large strongly correlated systems. DC spin-projection calculations based on Löwdin's method are performed on a π-conjugated polyene radical cation exhibiting severe spin contamination. The resulting DC energies closely reproduced those obtained from the standard (non-DC) spin-projected calculations. Additionally, the DC-based approximate spin-projection method and projected generator coordinate method were applied to a paddlewheel-type dichromium(II) complex and poly-phenoquinodimethane, respectively, demonstrating the applicability of the proposed approaches to systems with static correlation.
Takeshi Yoshikawa, Sousei Kasaya, Masatsugu Nishida et al.· Journal of Chemical Theory a...· 0 citations
Cis-stilbene exhibits ultrafast excited-state dynamics in solution, where photoisomerization and photocyclization compete on the picosecond timescale and are strongly influenced by solvent polarity. Despite extensive experimental studies, the molecular-level mechanism by which solvent environments control nonadiabatic relaxation and product branching remains unclear. Here, we perform QM/MM surface-hopping ab initio molecular dynamics simulations based on MRSF-TDDFT to investigate the excited-state dynamics of cis-stilbene in explicit nonpolar (n-hexane) and polar (methanol) solvents. The simulations qualitatively reproduce the experimental solvent trend, including the shorter excited-state lifetime and increased trans-isomer yield in methanol. Trajectory analyses revealed that twisted structures formed along the torsional coordinate exhibit enhanced dipole moments due to charge localization character in the S1 state. While this dipole enhancement occurs in both solvents, only in polar methanol does electrostatic stabilization significantly reduce the S1-S0 energy gap, thereby promoting nonadiabatic transitions over a broader configurational space. These results demonstrate that solvent polarity reshapes the excited-state branching landscape of cis-stilbene by modulating nonadiabatic transitions through electrostatic stabilization of charge-separated twisted geometries. The present study provides direct computational insight into solvent-controlled photoreaction dynamics and highlights the importance of explicit solvent effects in ultrafast processes in solution.
Yusuke Minegishi, Satoi Wada, Takuro Tsutsumi et al.· Journal of Chemical Physics· 0 citations
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