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**