Thanks Bert. With the method recommended by you I am getting MP2 energies like the following for doublet of heavy metal trimer within D3h symmetry. Can you please help me out where am I going wrong?
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SCF energy -164.975490357105
Correlation energy 45382298927.220947265625
Singlet pairs 0.000000000000
Triplet pairs 0.000000000000
Total MP2 energy 45382298762.245460510254
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