Simulation of Excited States and Solvatochromic Spectral Shifts

Simulation of Excited States and Solvatochromic Spectral Shifts 

The combined QM/MM model has been recently extended to simulation of the solvent effects on excited states and electronic spectroscopy by using an MO configuration interaction (Cl) formalism. 3 Using the Hartree—Fock wavefunction generated by the procedure described above as the reference state, the Cl wavefunction for the QM region can be constructed  

The total ground state (g) energy of a condensed phase system in such a Cl representation is  

A straightforward application of the combined QM-CI/MM model is to examine the solvent effects on absorption or emission spectra of chromophores in solution, whereas the method is of general use for studying photochemical reactions and electron transfer processes in solution and in enzymes. The solv-atochromic spectral shift is defined as the difference between the excitation energy of the solute in solution and tba In be gas phase 

However, description of the mutual solute—solvent polarization effects for both the ground and the exdted states within molecular orbital theory is not straightforward, because it requires SCF calculations of the solvent-induced dipoles in each step of the HF—SCF interaction. A simplification can be made by treadng the solvent molecule with fixed, efifeaive partial charges (thus ignoring explicit solvent polarization effects). In this case, = Eg, and the calculated spectral shift may be further decomposed into two terms  

---