| 10:00:56 PM PDT - Tue, Apr 30th 2013 |
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Hi,
I have run a Vibrational Self Consistent Field calculation on a CO2 molecule with NWChem. The resulting potential energy surface stored in the co2.coup file (I chose a pair mode coupling) surprised me, because it used only 3 normal modes of the molecule. That is, the PES looked like this:
ModeA ModeB ProgressionAlongMode Energy .....
1 2
1 2
...
1 3
1 3
...
2 3
2 3
...
End of file
But the CO2 molecule has 4 normal modes! There is one symmetric stretch, one asymmetric stretch and two bending modes (call one bend_X and the other bend_Y). bend_X and bend_Y (mode 3 and mode 4) are degenerate. But the coupling of modes 3 and 4 should still be included in the PES, because progression along both mode 3 and 4 at the same time corresponds to a nuclear configuration which is not explored by the PES NWChem has output. To clarify:
Energy(Q1=0,Q2=0,Q3,Q4) is not equal to Energy(Q1=0,Q2=0,Q3=0,Q4)
where the Qn here signify progression along the nth normal mode (Qn =0 means at equilibrium for that mode)
I suspect this is a problem with my input file geometry specification:
"start
restart
title co2
geometry noautoz units angstrom
C 0.00000000 0.00000000 0.00000000
O 0.00000000 0.00000000 1.18792006
O 0.00000000 0.00000000 -1.18792006
end
basis
C library sto-3g
O library sto-3g
end
vscf
coupling pair
ngrid 25
iexcite 1
end
task scf vscf"
Any ideas why the PES output is lacking a normal mode and how I could get NWChem to output that information?
Thanks,
Andrew
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