Qmmm freq

From NWChem

Potential Energy Surface Analysis: Optimization | Transition States | Hessians and Frequency | NEB |

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Setup

QM/MM hessian and frequency calculations are invoked though the following task directives

task qmmm <qmtheory> hessian

or

task qmmm <qmtheory> freq

Only numerical implementation are supported at this point and will be used even in the absence of "numerical" keyword. Other than standard QM/MM directives no additional QM/MM input is required for default hessian/frequency for all the QM atoms. Using region keyword(first region only), hessian/frequency calculations can also be performed for classical solute atoms. If only classical atoms are involved density keyword can be utilized to enable frozen density or ESP charge representation for fixed QM region. Hessian/frequency calculations for solvent are not possible.

Examples

Example of QM/MM frequency calculation for classical region

This example illustrates QM/MM frequency calculation for Ala-Ser-Ala system. In this case instead of default QM region (see prepare block), the calculation is performed on classical solute part of the system as defined by region directive in QM/MM block. The electrostatic field from fixed QM region is represented by point ESP charges (see density directive). These ESP charges are calculated from wavefunction generated as a result of energy calculation.

 memory total 800 Mb
 start asa
 permanent_dir ./perm
 scratch_dir ./data
 #this will generate topology file (asa.top), restart (asa_ref.rst), and pdb (asa_ref.pdb) files.
 prepare
   source asa.pdb
   new_top new_seq
   new_rst
   modify atom 2:_CB quantum
   modify atom 2:2HB quantum
   modify atom 2:3HB quantum
   modify atom 2:_OG quantum
   modify atom 2:_HG quantum
   center
   orient
   solvate
   update lists
   ignore
   write asa_ref.rst
   write asa_ref.pdb   # Write out PDB file to check structure
 end
 task prepare
 md
   system asa_ref
 end
 basis "ao basis"
   * library "6-31G*"
 end
 dft
  print low
  iterations 500
 end
 qmmm
 region mm_solute
 density espfit
 end
 # run energy calculation to generate wavefunction file for subsequent ESP charge generation
 task qmmm dft energy
 task qmmm dft freq