LNO-MP2 calculations from LNO-CCSD(T)

2 years 1 week ago #1100 by cnaim94
Dear MRCC developers,

I am making some LNO-CCSD(T) calculations and I want to compare the results also with LNO-MP2 ones. In my input, I am using the keywords calc=LNO-CCSD(T) and lcorthr=tight. From the output I can read that I have the values LMP2, are they actually proper LNO-MP2 calculations? I am saying so because if I directly run an LNO-MP2 calculation changing the corresponding parameters of the LNO decomposition I don't have exactly the same energies. Could you please let me know if I am not understanding properly what are the LMP2 energies or if I am making some mistakes in the keywords of my LNO-MP2 calculations?
These are the keywords that I change in the LNO-MP2 inputs in order to reproduce the LMP2 values:

Best Regards,
Carmelo Naim

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2 years 1 week ago #1101 by nagypeter
Dear Carmelo,

There is indeed a proper LMP2 correlation energy computed as an intermediate result of LNO-CCSD(T) calculations, which could be used by itself.

With the localcc=2018 version there is a small difference between the LMP2 correlation energy obtained via calc=lmp2 or as an intermediate of LNO-CCSD(T).
The "LMP2 correlation energy [au]:" line of the output of a calc=LNO-CCSD(T) job gives an LMP2 correlation energy as discussed in Ref [30] of the manual with minor changes reported in Ref [37]. The difference compared to calc=lmp2 is that the NAF approximation is not employed in the extended domains of an LNO-CCSD(T) calculation, while by default, the NAF approach is still turned on for LMP2 to stay consistent with Ref [30]. (This will be unified in a future release and a newer localcc option.)

So you should be able to get the "LMP2 correlation energy [au]:" value of an LNO-CCSD(T) calculation by setting

(Please, also note that natural orbitals are not employed in our method at the MP2 level, so there is no "LNO-MP2" method and some thresholds governing the LNO-CC calculations (e.g., lnoepso, lnoepsv) do not take effect for LMP2.)

Thanks for the question and let me know if you need more help.
Best wishes,

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