QM/MM hessian and frequency calculations

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](asa.top)), restart ([asa_ref.rst](asa_ref.rst)), and pdb ([asa_ref.pdb](asa_ref.pdb)) files.  
 prepare  
   source [asa.pdb](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](asa_ref.rst)  
   write [asa_ref.pdb](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