OPA spectra calculation with VG approximation

Questions
#1

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?

Thanks.
Paulami.

#2

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.

#3

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.

#4

(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.