# hessian

`psi4.driver.``hessian`(name, **kwargs)

  Function complementary to `frequency()`. Computes force constants, deciding analytic, finite difference of gradients, or finite difference of energies.

  Returns: | `Matrix` – Total non-mass-weighted electronic Hessian in Hartrees/Bohr/Bohr.
  Returns: | (`Matrix`, `Wavefunction`) – Hessian and wavefunction when return_wfn specified.
  Examples: |

      >>> # [1] Frequency calculation without thermochemical analysis
      >>> hessian('mp3')

      >>> # [2] Frequency calc w/o thermo analysis getting the Hessian
      >>> #     in file, core.Matrix, and np.array forms
      >>> set hessian_write on
      >>> H, wfn = hessian('ccsd', return_wfn=True)
      >>> wfn.hessian().print_out()
      >>> np.array(H)