Local configuration interaction single excitation approach: Application to singlet and triplet excited states structure for conjugated chains

A local configuration interaction single (LCIS) excitations approach is implemented and applied to investigate the electronic excited states of extended conjugated polyenes up to 400 carbons within the semi-empirical Pariser–Parr–Pople (PPP) model. Numerical results show that the LCIS approach can reproduce well the canonical CIS results with much less computational costs. The dimension of the Hamiltonian matrix scales linearly in LCIS approach, comparing to quadratic scaling with canonical orbitals. Calculations for the lowest singlet excitation energy and the singlet–triplet splittings up to system with 400 carbons are demonstrated to saturate at about 100 carbons for polyene chain.