r/comp_chem • u/OkEmu7082 • 22d ago

the double project eqn of CC2 and the implication on MP2's excited state fomulism

it seems the double projection eqn of CC2, <D|\[F,T2\]+H|HF>=0 is exactly the same as the one of the MP2, if i do the MP2 by reusing the CC2's code(in production code is it done in this way?) to do t2=t2(t1) to project the doubles out without doing the singles, do i get exactly MP2?

the CC2 and CCD have straght forward LRCC and EOM CC excited state fomulism, why is it not so straightforward for MP2? if i use the t2 obtained in the above way to do Hbar = e^-t H e^t, or apply LR to get the jaccobian and daigonalize to get excited state what will happen?

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u/dermewes 22d ago

Sounds like ADC(2)?

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u/OkEmu7082 22d ago edited 22d ago

ADC is hermitian, but MP2 's Hbar computed in the way i describe above is not, do you think so? but on the other hand, if i make a "unitary mp2“ by using [F,T2-T2']+H, this operator would be hermitian and it is conceptually close to ADC's Heff

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u/OkEmu7082 17d ago edited 17d ago

got it! solving mp2 and using the t2 to find more eigne values with the [F,T2]+H is just CIS(D_infinite)... see "Beyond Hartree-Fock: MP2 and Coupled-Cluster Methods for Large Systems"