[molpro-user] DIATOMIC routine and harmonic potential

[Kucukbenli Emine]

Dear Molpro users and developers,

As a new Molpro user I am experimenting with several features. Currently I want to understand better how to extract the spectroscopic parameters using "Diatomic" routine. To test the basics, I made the following table of bond length versus energy for a hypothetical molecule with perfectly harmonic potential: reduced mass of 1u, equilibrium bond length of 1.6 angstrom, equilibrium energy of 1eV and a spring constant of 1eV/Angstrom^2:

      DIST     ENRGY
    1.5000   1.0050000000
    1.5200   1.0032000000
    1.5400   1.0018000000
    1.5600   1.0008000000
    1.5800   1.0002000000
    1.6000   1.0000000000
    1.6200   1.0002000000
    1.6400   1.0008000000
    1.6600   1.0018000000
    1.6800   1.0032000000
    1.7000   1.0050000000

The statistics subcommand of table command gives me
===========
 Quadratic fit to 3 lowest: x0/y0/hessian
             1.6000000000
             1.0000016138
             1.0000000000
===========

Doing the unit conversion I find that the harmonic frequency of this molecule should be 3276 cm-1. See the wolfram alpha link here:
http://www.wolframalpha.com/input/?i=sqrt%281+eV%2FAngstrom^2+%2F%281+u%29%29%2Fc+in+cm^-1

However the result of Diatomic is
========================================================================================================
 ENRE     RE(AU)   RE(A)   BE       AE      WE     WEXE   WEYE   DE    D0      SIG
 ========================================================================================================
 1.0000000  3.0236  1.6000   6.585 -0.0956  2720.23   0.00  0.00  0.00  0.00  0.4812875E-16
 ========================================================================================================

As you see, the harmonic frequency w_e is found to be 2720.
I do not understand where this difference comes from.  I would greatly appreciate your help if you could point out what I am missing here.

The input file:
======================
***,harmonic pot. k=1
redm=1.0
ii=0
dr=0.02
DO r=1.5,1.7,dr,ANG
ii=ii+1
dist(ii)=r
enrgy(ii)=1+0.5*(r-1.6)*(r-1.6)
ENDDO
{table, dist, enrgy;  digits, 4,10 ;statistics; diatomic,degree=2, MASS=redm}
=================

Thank you,
emine kucukbenli, postdoc @ theos, epfl, switzerland


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