To calculate frequencies for active atoms only set
set gen_hess:actlist <list of atoms>
I have tried this for an H2CO-H2O complex calculating the frequencies for just the water molecule and this seems to work fine. The only thing that seems dubious are the results for the projected frequencies. In cases they are very different from the frequencies obtained for the whole molecule.
Input:
echo
start h2co_h2o_dat
geometry
h 0.00000000 -0.92494809 -2.76629063
h 0.00000000 0.92494809 -2.76629063
c 0.00000000 0.00000000 -2.18887270
o 0.00000000 0.00000000 -1.00283061
h 0.00000000 -0.74265427 1.42611142
h 0.00000000 0.74265427 1.42611142
o 0.00000000 0.00000000 2.01432013
end
basis
* library 6-31g*
end
set gen_hess:actlist 5:7
task scf freq numerical
The unprojected frequencies obtained were:
All atoms Water only
1 -144.223 -142.345
2 -18.688
3 -14.172
4 -10.643
5 3.832
6 17.860
7 34.616
8 49.839
9 77.986 99.454
10 113.074 88.753
11 146.008
12 384.063 380.659
13 1340.106
14 1383.798
15 1678.948
16 1839.585 1840.264
17 2022.956
18 3173.133
19 3253.921
20 4088.318 4088.315
21 4188.738 4188.737
I think there should be at least 2 more sensible frequencies for the water only case but I cannot identify those from the frequencies and intensities. The projected frequencies are in places very different.
Huub