Analytical energy gradient of the symmetry-adapted-cluster configuration-interaction general-$R$ method for singlet to septet ground and excited states

A method of calculating analytical energy gradients of the singlet and triplet excited states, ionized states, electron-attached states, and high-spin states from quartet to septet states by the symmetry-adapted-cluster configuration-interaction general-$R$ method is developed and implemented. This method is a powerful tool in the studies of geometries, dynamics, and properties of the states of molecules in which not only one-electron processes but also two- and multielectron processes are involved. The performance of the present method was confirmed by calculating the geometries and the spectroscopic constants of the diatomic and polyatomic molecules in various electronic states involving the ground state and the one- to three-electron excited states. The accurate descriptions were obtained for the equilibrium geometries, vibrational frequencies, and adiabatic excitation energies, which show the potential usefulness of the present method. The particularly interesting applications were to the $C′ 1Ag$ state of acetylene, the $A 2Δu$ and $B 2Σu+$ states of CNC and the $B14$ and $a 4Πu$ states of $N3$ radical.