# How to reproduce the energy of optimized geometrical structure?

When I used PSI4 to run a molecular structure optimization of H2O, the total energy of optimized molecule is -75.985359. But when this geometrical structure data is used for an energy calculation, it gives a slightly different result: -75.6327814.

I am quite inexperienced in PSI4, I have attached both inputs here, hope someone can point out which part was done incorrectly.

Step one: Run an geometry optimization of H2O using these parameters:

psi4.set_memory(‘2000 MB’)
h2 = psi4.geometry("""
O
H 1 0.96
H 1 0.96 2 104
“”")

psi4.set_options({‘basis’:‘6-31g’, ‘scf_type’:‘pk’, ‘g_convergence’:‘GAU_VERYTIGHT’,‘reference’:‘uhf’})
energy,wfn = psi4.optimize(‘scf’,return_wfn=True)

Total energy = -75.985359

Step two: The optimized geometry from step one, was used in energy calculation, using same parameters:

memory 600 mb

molecule h2o {
O 0.000000000000 0.000000000000 -0.112965651246
H 0.000000000000 -1.483749390906 0.896423431217
H 0.000000000000 1.483749390906 0.896423431217
}

set basis 6-31g
set scf_type pk
set g_convergence GAU_VERYTIGHT
set reference uhf
energy(‘scf’,return_wfn=True)

Total energy = -75.6327814025212319

The output is printed in Bohr, if you add `units Bohr` to the molecule specification you get:

UHF Final Energy: -75.98535917636522

Thanks for the answer, it solved the problem.