Dear all,
i am trying to optimise the zwitterionic form of glycine in the gas phase. As the ionic form is not stable in gas phase, I am constraining the 3 N-H distances to their initial values. Without these constraints the geometry relaxes to the neutral glycine. Unfortunately, the optimisation does not converge and oscillates. I figured out that using opt_coordinates both
helps the optimisation in the unconstrained case, as in internal coordinates the O-H bond is probably missing, but it does not help in case of the constrained optimisation.
memory 20 GB
molecule GlycinZwitter {
0 1
O 5.08692 1.03668 -0.91140
C 4.57228 1.85982 -0.08309
C 3.09589 1.47366 0.16704
O 5.01218 2.86391 0.51139
N 2.72038 0.33365 -0.75446
H 3.67355 0.08337 -1.12936
H 2.41238 2.29845 -0.04728
H 2.96232 1.10961 1.18889
H 2.18794 0.64400 -1.56732
H 2.33753 -0.48499 -0.28551
}
set {
basis cc-pvtz
scf_type df
mp_type df
reference rhf
geom_maxiter 150
opt_coordinates both
# full_hess_every 10
}
set optking {
fixed_distance = ("
5 6 1.02
5 9 1.02
5 10 1.02
")
}
#optimize("omp2.5")
optimize("scf")
#frq, wfn =frequency("omp2.5", return_wfn = True)
#molden(wfn, "GlycinZwitter")
The problem in this thread seems very similar but i was also in bad look using them.
Is there anything i could improve to get rid of these oscillations or is the system just difficult and i need to live with hundreds of iterations?
Best wishes
Phillip