ORCA Max Plank Institute
ORCA is a multi-purpose quantum-chemical software package. It features a wide variety of methods ranging from semi-empirical methods to density functional theory to correlated single- and multi-reference wave-function based methods. Environmental as well as relativistic effects can be taken into account.
ORCA is the leading software tool in the calculation of spectroscopic properties of open-shell molecules.
RKS or UKS This are the flags to indicate a DFT calculation. it stands for, Restricted Kohn Sham for closed shell systems or Unrestricted Kohn Sham for open shell systems.

PBE Density functional to calculate the exchange correlation energy. Altenatively could be PBE0, B3LYP 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.

#ORCA Basic Input DFT calculation
! UKS 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
 Symbol   X1  Y1  Z1
 C        X2  Y2  Z2
 H        X3  Y3  Z3
 Pt       X4  Y4  Z4
*
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