QM/MM hessian and frequency calculations¶
QM/MM hessian and frequency calculations are invoked though the following task directives
task qmmm `<qmtheory>` hessian
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.
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