Energies and derivative couplings in the vicinity of a conical intersection. II. CH2(2 3A″,3 3A″) and H2S(1 1A″,2 1A″), unexpected results in an ostensibly standard case

The ${\text{2\hspace{0.17em}}}^3{A}^{\mathrm{\prime \prime }}{\text{−3\hspace{0.17em}}}^3{A}^{\mathrm{\prime \prime }}$ and the ${\text{1\hspace{0.17em}}}^1{A}^{\mathrm{\prime \prime }}{\text{−2\hspace{0.17em}}}^1{A}^{\mathrm{\prime \prime }}$ seams of conical intersection in ${\mathrm{CH}}_{\mathrm{2}}$ and ${\mathrm{H}}_{\mathrm{2}}\mathrm{S},$ respectively, are considered. The nuclear coordinate dependence of the seam of conical intersection, the energy of the lower adiabatic potential energy surface along closed loops containing the conical intersection, and the nonremovable part of the derivative coupling in the region contained within the closed loops are studied. The energetics and derivative couplings in the vicinity of the conical intersections are analyzed in terms of the characteristic parameters of a conical intersection, determined at the configuration interaction level using analytic gradient techniques. The characteristic parameters are found to predict, in a qualitative manner, the energetics at moderate distances from the conical intersection. Loops containing the conical intersection that exhibit and do not exhibit the geometric phase effect are considered. An unusual trifurcation of the $C_{\text{2v}}$ seam of conical intersection in ${\mathrm{CH}}_{\mathrm{2}}$ into a $C_{\text{2v}}$ branch and ${\text{2C}}_s$ branches is discovered. For ${\mathrm{H}}_{\mathrm{2}}\mathrm{S},$ over the range of nuclear coordinates considered, only points of conical intersection with $C_{\text{2v}}$ symmetry were found.