[molpro-user] IR intensity of linear molecule

[ZOUWL]

Dear developers,

In the case of linear molecule (no matter symmetry is used or not), if
analytic gradient is not avilable, the IR intensity of bending mode become
to zero. The problem is in the Dipole Moment Derivatives: all the values in
the X and Y components are zero by mistake.

For example, the bending mode Pi_u of acetylene (HCCH) is IR active, but
the following calculation leads to a zero intensity. However, if the linear
symmetry is broken by a small distortion, the IR intensity of Pi_u is
correct.

***,C2H2

basis=3-21g

geometry={
C
X 1 1.
C 1 R1 2 90
X 3 1. 1 90 2 180
H 1 R2 2 90 3 180
H 3 R2 4 90 1 180
}

R1=1.21 angstrom;
R2=1.06 angstrom;

rhf;

ci;

{optg;active,R1,R2;}

{frequencies,print=1;print,hessian;}
---

Best regards,
Wenli
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