Thanks for your kindly reply!

1a. Sorry I don’t know much of the `normal ordered Hamiltonian`

, but I suppose that is not what I’m looking for. The expression I’m using is eqn(2) in paper, which contains integral using spin orbitals. So I thought the coefficients might change depending on the wavefunction I have.

1b. See the below descriptions about what I mean as using MP2 as reference state.

1c. I know I can use MP2 result to get a reasonable initial guess to CC coefficients. And can probably truncate number of double excitations based on `t^ij_ab`

. So I do want to know what are significant double excitation terms and what their initial values should be using PSI4. And additional question is if I can simply choose first few significant double excitation terms as truncation.

2a. Sorry for the confusion. For clarity, there should have a comma between `is`

and `are. Basically I was just asking how to pull out the significant double excitation determinants and its coefficient.

2b. Thanks!

2c. Sorry, I got sloppy there. I corrected indices in my originally post.

Ok. At this stage, I feel it is necessary to describe what I’m actually up to. I’m trying to simulate quantum chemistry using quantum computer using VQE (variational eigensolver) algorithm. So I want PSI4 to be part of the tool chain of some quantum computing process.

I don’t know if you are familiar with VQE, but actually it is just using quantum computer to simulate UCCSD with `e^{T-T*}|ref>`

, where `T = T1 + T2`

and in second quantized form with coefficients to be optimized. See paper here.

This is what I want from PSI4:

- A Hamiltonian in second quantized state in eqn(2) whose coefficients are given in eqn(3) and eqn(4)
- A reference state that in second quantized form that can be populated as using
`1^ 2^ 3^... |vac>`

in eqn(28)
- Being able to truncate
`T2`

term and being able to initialize `T`

terms with MP2 amplitudes .

So I know how to do this using `|HF>`

, as it naturally satisfies eqn(28). My question is, can I use |MP2> as similar product state and the reference state of UCCSD?