#1 Thu Jan 04, 2018 5:21 pm
Registered: Jan 2018
I want to calculate the singlet and triplet excited states of CH3I using EOM-EE-CCSD method. The results are so weird that I cannot explain.
Here is my input.
H 0.7515158257 -0.6022789095 -1.8859686281
C 0.0000275280 -0.0000480643 -1.3297166588
H 0.1457682315 0.9524366931 -1.8856497472
H -0.8975405428 -0.3496077564 -1.8858774957
I -0.0000005683 0.0000001805 0.1707858230
I used the geometry I got from Molpro, and the same basis set. Firstly, the Hartree-Fock energy I got from Molpro is -334 a.u., but q-chem returned me -2700 a.u., which I don't understand. Secondly, when I calculate the excited states, the excitation energy is negative for the first singlet and triplet excited state.
Does anyone have any sense what might be the reason for those two strange problems?
#2 Fri Jan 19, 2018 11:48 am
Registered: Oct 2017
This clearly indicates that SCF has converged to a wrong solution. I run this calculation and confirm that the singlet excitation energy is negative. This is a good test to check if SCF converges to the global minimum, but EOM-EE is an overshot for this purpose - CIS should be enough. It looks like the default guess is wrong. From CIS calculation the leading excitation amplitude to the lowest singlet corresponds to HOMO-LUMO excitation. Therefore, you can read orbitals as a guess and re-populate them. I run it with MOM_START=1, but it did not work. Also, start from the triplet state is worth to try.
Too negative energy may indicate about a problem with ECP. Could you attach the output from Molpro?