initial state in the "toExpH" function

Question

Dear itensor-developer,

In the example http://itensor.org/docs.cgi?page=formulas/tevol_mps_mpo
the "toExpH" function is used after defining the MPS to evolve.

auto psi = MPS(state);

but one uses

 auto expH = toExpH<ITensor>(ampo,tau);

rather than, for example, auto expH = toExpH(ampo,psi, tau);

what if I define psi1 and psi2 and after that use toEXPH? what is the initial state in this case?

Thank you in advance

Answer 1

Hi,
toExpH only converts ampo object to exp(−tauH), you can use exactApplyMPO or fitApplyMPO to compute exp(−tauH)|psi>

e.g. psi1 is the initial state:
auto expH = toExpH<ITensor>(ampo,tau);
psi1 = exactApplyMPO(expH,psi1,args);