Dear
all, I am
recently learning excited state calculations (TDDFT or EOM-CC). When I compared
the results using basis sets cc-pvtz or aug-cc-pvtz, I have following
observations (excitation energies were always calculated up to 9.92eV,
which is the limit I can reach experimentally using vacuum
uv): 1) with
aug-cc-pvtz, more excitations (for transitions <9.92eV) were found.
From
what I read the reason seems to be that a lot of Rydberg transition can only be
found with diffuse functions. So a lot of Rydberg transitions are missing when
using cc-pvtz. Is this explanation correct? 2) with
aug-cc-pvtz, the total sum of oscillator strengths (for all transitions
<9.92eV) increase significantly as well. There is
sum rule that \sigma_j f_{ij}=1 and \sigma_{ij} f_{ij}=N (total number of
electrons). Ideally if all transitions were found, sum of f should be the same.
So I think if the number of transitions is big enough, the sum of f should tend
to be the same. For many molecules I calculated, the number of transitions is
several hundred. So why the sum of f with aug-cc-pvtz is always much bigger than
the one with cc-pvtz? I am interested in this question because I want to
correlate the total oscillator strength to experimental total response from
vacuum uv, but I not sure which calculated results I should trust
more. Thank
you very much! M.M |