Low scaling excited state calculation using block interaction product state

We develop an automatic and efficient scheme for the accurate construction of the bases for excitonic models, which can enable "black-box" excited state structure calculations for large molecular systems. These new and optimized bases, which are named as block interaction product state (BIPS), can be expressed as the direct products of the local states for each chromophore. Each chromophore's local states are selected by diagonalization of its reduced density matrix (RDM), which is obtained by quantum chemical calculation of the small subsystem composed of the chromophore and its nearest neighbors. We implemented BIPS framework with fragment-based calculations considering 2-body and 3-body interactions. Test calculations for 8 different molecular aggregates demonstrate that this framework provides accurate description of not only the excitation energies, but also the first-order wavefunction properties (dipole moment and transition dipole moment) of the low-lying excited states at a low-scaling computational cost.