OPA spectra calculation with VG approximation

Hi everyone,

I was trying to simulate OPA spectra of flavin molecule and its five oxidation states in gas-phase and in PCM solution. The molecules are in S0 minima i.e. in equilibrium geometry and the excited states equilibrium geometries also are similar as S0 minima. But the spectra I am getting, is always 50-100 nm blue shifted with respect to experiment as well as the spectra calculated by ezFCF at same level of theories (TDDFT-b3lyp/cc-pVTZ). Can anyone tell us please What is/are the reason(s) for blue shift though the molecules are in equilibrium geometry?


If we take an absorption band at ~400 nm, or 3.1 eV (in theorist-approved units), then a 50 nm blueshift amounts to 3.5 eV. That’s on the larger side for TDDFT valence excitation energies errors but certainly not unheard of; 100 nm shift (=1eV) is a bit harder to swallow, especially if it’s shifting to higher energy. Are you using the non-eq solvent corrections for vertical excitation energies? Could make a large difference, esp. since some of those oxidation states are charged, yes? Also no guarantee that B3LYP is the right answer to all problems.

Thanks for your reply. Yes, the oxidation states are charged but our concern is we have simulated the spectra in gas-phase with TDDFT-b3lyp/cc-pvtz level of theory with ezFCF and that was completely matched with experiment. But with the same level of theory in gas-phase also, VG is giving spectra which are ~50 to 100 nm shifted in every cases. We cannot understand what are the problems going on here.

(a) what is VG?
(b) what specific options are you using to model solvation?

I’ve certainly seen cases where the non-eq continuum solvation correction approaches 1 eV for a charged solute.