Hi @Kaushik ,
I recommend that you go to the docs for qchem.molecular_hamiltonian.
In the SSVQE code which I shared here you will notice that we call qchem.molecular_hamiltonian and we set a number of active electrons and active orbitals.
Let’s imagine you have a molecule with 10 electrons. Most likely the first 6 electrons will stay in the lowest-energy orbitals, so you can set your active electrons as 4. You can then decide how many active orbitals you want to have. It makes sense to have a maximum of 4 active orbitals because most likely the electrons will need a lot of extra energy to occupy higher-energy orbitals, so it’s unlikely to be a natural state for that molecule.
Conclusion, instead of simulating all configurations of all 10 electrons in a ton of orbitals you’re restricting your problem to just a set of combinations that will most likely give you the answer you want.
For some very big molecules however you may still need too many qubits to simulate active electrons in the active orbitals, and a further reduction in the size of the simulation may compromise accuracy. There’s a reason why we’re excited about building actual quantum hardware! We hope this will allow us to tackle these complicated simulations in the future.