Subject: numerical force field!

From: <[email-masked]>
Date: Wed, 25 May 1994 09:28:55 +0000 (O)

Hi CCL:

I calculted the force field for a five membered heterocyclic molecule at 
the SCF and MP2 levels of theory using the 6-31G** basis set. I also 
calculated the ir abosrtion intensities, after scaling the force field. 
I found that, for the in-plane vibrational modes, the agreement 
between the experiemtnal and calculated ir absorption intensities at the 
MP2 level is in excellent agreement with the experiemtnal intensities. Not 
only weak and strong bands are correctly preidcated but also the reletive 
intensities of most of the weak and strong bands are almost correctly 
predicated. The SCF calculated intensities were also good but not as 
good as those calculated at the MP2 level. But for the out-of-plane 
vibrational modes the agreement with the MP2 calculated intensities is 
poor and the SCF calculated intensities are significantly much better. 

My question is simply is there any explanation for this? There is one 
difference so. That is the MP2 force field and dipole derivative tensors 
were calculated numerically and those calculated at the SCF level were
calculated analytically. Could be this the reson! To make thing clearer, 
I am including the frequencies anf ir absortion intensities, experimental 
and at the SCF and MP2 levels of theory using the 6-31G** basis set.

in-plane vibational modes

  Experimental     MP2/6-31G**      SCF/6-31G**
   
  freq.   int.     freq.  int.       freq.  int.

  1560    40       1558     40        1567    143
  1432    85       1439    154        1438    395
  1373    34       1365     31        1369      7
  1217    19       1211     23        1215     75
  1128    75       1136    145        1125    172
  1089    28       1093     30        1083    111
  1021    21       1031     60        1023     33
   917    49        918    115         919    218
   -      -         900     10         907     59
   856    71        854    206         857    447

out-of-plane vibational modes

  (889)   30        841    156         925    156
  (792)    -        806    672         900      2
   764   215        757      2         768    660
   632    19        632    143         636     33
   595    66        592     42         585    365

Adel El-Azhary
Cairo Univerity