Previous
Index
Next
Geometry optimization

One of the most important calculation is to perform a geometry relaxation. To find the geometry with the least energy. From here we can start to do all the other properties we want to calculate.

Let's find the bond length for the hydrogen molecule. First you have to specify which theory the code is going to use. In this tutorial we will only use DFT.

Then you have to specify the type of coordinates you will use: XYZ type or Internal coordinates.

RKS or UKS
Kohn Sham keyword allows you to use DFT.
RKS: Restricted KS for closed shell systems.
UKS: Unrestricted KS for open shell systems.

XC functional
Functional to calculate the exchange correlation energy: PBE, B3LYP, PW91, RPBE, TPSS0, M06, M062x etc...

Opt Default Single Point. Indicates a geometry optimization calculation.

NumFreq Numerical Derivatives. Alternative AnFreq that calculates the faster analytical frequencies but no the dipole changes.

[emma@pople]$ vi geom_opt.inp


#ORCA Basic Input DFT calculation
! RKS PBE TZVP
! PMODEL  #Initial density guess
! XYZfile  #Type of coordinates
! TightSCF  #SCF convergence criteria
! PAL8  #Number of core processors

! Opt  #Geometry optimization
! NumFreq  #Numerical Frequency
! NumFreq  #Vibrational analysis.
%method
  Grid  2 #110 grid points for the SCF
  FinalGrid  4 #302 grid points
end

#Type  Charge   Multiplicity
* xyz    0   1
 H        0.0  0.0  0.5
 H        0.0  0.0  0.5
*
[emma@pople]$/path/to/orca geom_opt.inp


Alternative for torque users

[emma@pople]$qsub geom_opt.inp