Transfer of Frequency-Dependent Polarizabilities: A Tool To Simulate Absorption and Circular Dichroism Molecular Spectra.

Absorption and circular dichroism spectra reveal important information about molecular geometry and electronic structure. For large molecules, however, spectral shapes cannot be computed directly. In the past, transition dipole coupling (TDC) and related theories were proposed as simplified ways of calculating the spectral responses of large systems. In the present study, an alternative approach better reflecting the chemical structure is explored. It is based on the transfer of complex frequency-dependent polarizabilities (TFDP) of molecular fragments. The electric dipole-electric dipole, electric dipole-electric quadrupole, and electric dipole-magnetic dipole polarizabilities are obtained separately for individual chromophores and then transferred to a larger system composed of them. Time-dependent density functional theory and the sum over states methodology were employed to obtain the polarizability tensors of N-methylacetamide, and porphyrin molecules were chosen for a numerical test. The TFDP fails for charge-transfer states and close chromophores; otherwise, the results suggest that this method is capable of reproducing the spectra of large systems of biochemical relevance. At the same time, it is sufficiently flexible to account for a wide range of transition energies and environmental factors instrumental in the modeling of chromophore properties. The TFDP approach also removes the need for diagonalization in TDC, making computations of larger molecular systems more time-efficient.