[molpro-user] MP2 pair energies

[Hans-Joachim Werner]

Dear Matthias,
sorry that this is somewhat confusing. The scaling factors of Grimme apply to the „parallel“ and „antiparallel“ spin contributions (in a spin-orbital formulation), but these are not identical to what one usually denotes as „singlet“ and „triplet“ pairs (which means that the 2 external electrons are coupled to a singlet or a triplet). In the closed-shell case the triplet energy is the parallel contribution times 3/2 (since one third comes from the T0 contribution, which depends on the antiparallel spin contribution). The singlet contribution is the rest. So in your example

 esing=  -0.126127623239
 etrip=  -0.076321972220
 ecorr=esing+etrip
 ps=1.2
 pt=1/3
 epara=etrip*2/3
 eanti=esing+etrip/3
 escs=epara*pt+eanti*ps

yields

 SETTING ESING          =        -0.12612762
 SETTING ETRIP          =        -0.07632197
 SETTING ECORR          =        -0.20244960
 SETTING PS             =         1.20000000
 SETTING PT             =         0.33333333
 SETTING EPARA          =        -0.05088131
 SETTING EANTI          =        -0.15156828
 SETTING ESCS           =        -0.19884238



Best wishes
Joachim
---
Prof. Dr. Hans-Joachim Werner
Institut für Theoretische Chemie
Universität Stuttgart
Pfaffenwaldring 55
70569 Stuttgart, Germany
e-mail: werner at theochem.uni-stuttgart.de




> Am 05.12.2017 um 22:02 schrieb Matthias Heger <heger at ualberta.ca>:
> 
> I am a bit confused about the MP2 pair energies reported in my calculations. Specifically, my attached example yields:
> 
>> SCS-MP2 correlation energy:           -0.198842375046   (PS=  1.200000  PT=  0.333333)
>> (...)
>> MP2 singlet pair energy              -0.126127623239
>> MP2 triplet pair energy              -0.076321972220
> 
> Scaling the pair energies manually and adding them yields a different energy (-0.176792... Hartree) than given above.
> 
> In addition, I tricked the SCS calculation to only pick up the singlet or triplet pairs with SCSFACS and SCSFACT. That yields SCS correlation energies different from the corresponding pair energies in the output.
> 
> Am I interpreting this wrong?
> 
> Best,
> Matthias
> 
> -- 
> Dr. Matthias Heger
> Department of Chemistry, University of Alberta
> 11227 Saskatchewan Drive NW
> Edmonton, AB T6G 2G2, Canada
> 
> Office: CCIS 4-151
> Lab: CCIS 4-108
> Email: heger at ualberta.ca
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