DFT Hess - Open-Quantum-Platform/openqp GitHub Wiki

An Examples of a Restricted Hartree-Fock(RHF) DFT Numerical Hessian calculation

[input]
system=
   O  -0.0000000000   0.0000000000  -0.0410615540
   H  -0.5331943294   0.5331943294  -0.6144692230
   H   0.5331943294  -0.5331943294  -0.6144692230
charge=0
functional=bhhlyp
basis=6-31g*
runtype=hess
method=hf

[scf]
type=rhf
multiplicity=1

[hess]
state=0

[input] Section

  • system: This is the coordination of your system, your system's coordination can be achieved through two primary methods for OQP. The first method employs the Standard Cartesian format, as illustrated in the provided example. The second method utilizes the .XYZ format. To leverage the .XYZ format, save your coordination details in this format within your input folder, for instance, as H2O.xyz. Subsequently, it can be easily applied by specifying system=H2O.xyz in your input file. Here is an example of .XYZ format:
3
symmetry c1
O        0.000000000      0.000000000     -0.041061554
H       -0.533194329      0.533194329     -0.614469223
H        0.533194329     -0.533194329     -0.614469223

  • charge: The total charge of the system. A value of 0 indicates that the molecule is neutral, with no net charge.

  • runtype: Set to hess, indicating that the calculation will focus on obtaining the Hessian matrix, which involves calculating second derivatives of the energy with respect to nuclear coordinates.

  • basis: The basis set used for the calculation, 6-31gs in this case. The 6-31gs is a split-valence basis set with polarization functions on heavy atoms, designed to provide a good balance between accuracy and computational cost. You can find all of the basis-sets supported by OQP within the basis_set folder.

Just for your information, you can see the 6-31g* basis set provided for Hydrogen and Oxygen atom within the GAMESS US format 
#----------------------------------------------------------------------
# Basis Set Exchange
# Version v0.9.1
# https://www.basissetexchange.org
#----------------------------------------------------------------------
#   Basis set: 6-31+G*
# Description: 6-31G + diffuse and polarization functions on heavy
#                    atoms
#        Role: orbital
#     Version: 1  (Data from Gaussian 09/GAMESS)
#----------------------------------------------------------------------



# HYDROGEN
S   H
      0.3349460434E-01       0.1873113696E+02
      0.2347269535E+00       0.2825394365E+01
      0.8137573261E+00       0.6401216923E+00
S   H
      1.0000000              0.1612777588E+00

# OXYGEN
S   O
      0.1831074430E-02       0.5484671660E+04
      0.1395017220E-01       0.8252349460E+03
      0.6844507810E-01       0.1880469580E+03
      0.2327143360E+00       0.5296450000E+02
      0.4701928980E+00       0.1689757040E+02
      0.3585208530E+00       0.5799635340E+01
L   O
     -0.1107775495E+00       0.1553961625E+02       0.7087426823E-01
     -0.1480262627E+00       0.3599933586E+01       0.3397528391E+00
      0.1130767015E+01       0.1013761750E+01       0.7271585773E+00
L   O
      0.1000000000E+01       0.2700058226E+00       0.1000000000E+01
L   O
      0.1000000000E+01       0.8450000000E-01       0.1000000000E+01
D   O
      1.0000000              0.8000000000E+00
  • functional: Specifies the DFT functional to be used. bhhlyp is a hybrid functional that combines Hartree-Fock exchange with B88 exchange and LYP correlation. It's known for its good performance in a variety of systems.
Just for your information, here are some more explanation on BHHLYP

The BHHLYP functional is unique in its approach to mixing exact exchange with DFT exchange-correlation terms. Specifically, it combines:

  • 50% of exact exchange from Hartree-Fock theory,
  • 50% of exchange from the B88 functional (developed by Becke in 1988),
  • and correlation from the LYP functional (developed by Lee, Yang, and Parr).

This particular combination is designed to balance the need for accurate exchange energy representation with the computational efficiency and additional electron correlation effects provided by DFT.

  • method: Indicates that the Hartree-Fock (HF) method will be used. However, the specification of a DFT functional (bhhlyp) suggests that the calculation will incorporate DFT principles, which is indicating a hybrid approach. In practice, HF calculations do not use a functional, so this setup is aim to use RHF as the basis for a DFT calculation, combining the two methodologies.

[guess] Section

  • type: The initial guess for the molecular orbitals. huckel suggests using a simple Hückel molecular orbital (HMO) theory-based guess, which is particularly useful for π-conjugated systems but can serve as a starting point for various molecular systems.

[scf] Section

  • multiplicity: A multiplicity of 1 indicates a singlet state, which is typical for the ground state of closed-shell molecules like water, implying all electrons are paired.

  • type: The rhf (Restricted Hartree-Fock) method is selected for the SCF calculations. RHF is appropriate for singlet state systems and assumes all electrons are spin-paired, providing a wave function that is an eigenfunction of the total spin operator.

image

[hess] Section

  • state: Specifies that the Hessian calculation will be performed for the first excited state.

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