Hello, I’ve been trying to run CCSD(T) analytical gradient calculation. Elements in the system include C H O. The input file is attached below. It only takes about 30s to calculate the single point energy of the UHF system, but it takes more than 90 minutes to calculate the analytical gradient. When it comes to the RHF system, the calculation will be much faster. Is this right? What is the reason for consuming such a large calculation time in calculating the analytical gradient?

Thanks!

#! Tests RHF CCSD(T)gradients

memory 24GB

molecule ben {

C 25.682000 7.531000 1.718000

H 28.000000 9.389000 1.768000

C 27.359000 8.534000 1.328000

H 26.707000 9.052000 0.621000

H 27.913000 7.702000 0.971000

O 25.022000 6.872000 2.343000

symmetry c1

}

set {

reference uhf

basis 6-31g**

MAXITER 999

r_convergence 1e-5

e_convergence 1e-4

d_convergence 1e-4

guess_mix = true

}

analytic, wfn = gradient(‘ccsd(t)’, return_wfn=True)

fchk_writer = psi4.FCHKWriter(wfn)

fchk_writer.write(‘cho.fchk’)