Possible bug in reported force constants after vibrational analysis
Leopold Talirz
leopold... at gmail.com
Fri May 23 14:11:56 UTC 2014
Dear developers,
during an MD tutorial using CP2K I was looking at the force constants k
reported after vibrational analysis. Below you find an example for the
water molecule
VIB| 1 2 3
VIB|Frequency (cm^-1) 1589.789609 3637.026185
3741.860069
VIB|Intensities 0.000000 0.000000
0.000000
VIB|Red.Masses (a.u.) 1.083688 1.044444
1.081600
VIB|Frc consts (a.u.) 0.017723 0.467889
0.542862
I thought that the force constants are defined such that k = mu * omega^2,
where mu is the reduced mass and omega the vibrational frequency (?). But
this is obviously not the case.
Looking into vibrational_analysis.F , the formula
konst(i) =
SIGN(1.0_dp,H_eigval2(i))*2.0_dp*pi**2*(ABS(H_eigval2(i))/massunit)**2*rmass(i)
for the force constant on line 406 (r14024) involves H_eigval2^2, which is
proportional to omega^4 instead of omega^2.
Is this a bug?
Best,
Leopold
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