Calculating The Excited State Properties Of Molecular Aggregates By Renormalized Excitonic Model

Due to the high power growth of computational costs with the size of calculated systems,the traditional quantum chemical method is difficult to be applied to the calculation of excited states of large systems.In recent years,the development of new low-scale electronic excited state methods has become one of the more active frontier directions in the field of theoretical chemistry.In the early stage,our research group implemented the renormalized excitonic model（REM）method at the ab initio level,which approximates the excited state of whole system to a linear combination of several sub blocks’（local）excited states.The method can quantitatively calculate the excitation energies of kinds of localized or delocalized low excited states of large systems with high accuracy,and the computational costs only increase linearly with the size of the systems.In this paper,the ab initio REM method is extended to the study of other properties related to wavefunction of excited state except the energy,such as the atomic charge population analysis and the calculation of the transition dipole moment.In the implementation,we use symmetric orthogonal block canonical molecular orbitals（SYM-BCMOs）to avoid the difficulty of the Hartree-Fock calculation for whole system when we use traditional orthogonal localized molecular orbitals（OLMOs）and the calculation involving in non-orthogonal basis when we use block canonical molecular orbitals（BCMOs）.In order to overcome the shortcomings of the accuracy of REM wavefunction,we have generalized the effective Hamiltonian theory to the effective operator scheme to achieve the high-precision calculation of the expected values of various mechanical quantities.In this paper,the ab initio REM method is tested at the CIS level.We calculated the ionization potentials（IPs）of one-dimensional ammonia and formaldehyde molecular chains,and the Mulliken charges of the molecular chains with a unit positive charge,then the excited energies and transition dipole moments of S₀→S₁ of one-dimensional ethylene and pyrrole molecular chains are also calculated.On the premise of reasonably dividing the system into blocks and selecting the reserved states,not only energy but also wavefunction properties,the calculated results are in good agreement with the CIS reference values,and the computational costs increase linearly with the number of monomers.The tests also show that the computational accuracy of REM is closely related to the block size and the preserved states selected.It is the first time to realize the calculation of wavefunction properties of the excited states by ab initio REM method,which provides a new efficient theoretical tool for the study of the photophysical and photochemical properties of practical chemical condensed phase systems of molecular aggregates.