Non-adiabatic coupling calculation

Questions
#1

Hi, I was wondering how to compute NACs between singlet states. I have checked this webpage, in which some examples are written for QChem: Q-Chem 5.0 User’s Manual : Nonadiabatic Couplings and Optimization of Minimum-Energy Crossing Points and it is a bit strange to me because to compute the NAC between S0 and S1 the keywords are:

$derivative_coupling
comment
1 3
$end

while to compute NAC between S1 and S2 it is:

$derivative_coupling
comment
1 2
$end

Is it correct? I would like to compute NACs for S1-S0, S2-S1 and maybe S2-S0, but I am not sure whether there were errors in the webpage I looked at.

thank you

#2

This is a typo in the manual (which I have fixed for the next release). In the $derivative_coupling section you specify which states you want, with 0 indicating the reference state and 1,2,3,… being the excited states. You then get the couplings between all pairs of states that are indicated in the $derivative_coupling section. This is clear if you look at the output, because the energies of the individual state are listed as Ei and Ej, e.g.:

 ---------------------------------------------------
             CIS Derivative Couplings
 ---------------------------------------------------
              between states 0 and 1
NCharge is 0
 get_w_dc time:  CPU 0.60 s  wall 0.14 s
 AOints 129 time:  CPU 0.94 s  wall 0.21 s
 DFT time  CPU 0.62 s  wall 0.12 s
 Ei = -78.585544
 Ej = -78.268659
 Ej-Ei = 0.316886

and in this particular example (which is Example 9-218 in the manual) you should be able to identify Ei as the ground-state energy (state index 0) and Ej and the S1 energy (state index 1). Output (including the derivative couplings that are printed next) is in atomic units.