Using A Quantum Chemistry DB in MCTDH Calculations
A Quantum Chemistry Database (QC DB) can be generated in a number of ways such as a DD-vMCG or DD-TSH simulation, or using the VCHam tool makedb to put together the results from quantum chemistry calculations. The QC DB can then be used to provide the potential energy surfaces and couplings in grid-based calculations in the following ways.
The basic rerpesentation of the surfaces is by Shepard interpolation through the points in the QC DB, with couplings in the diabatic picture provided by, e.g. propagation diabatisation or Procrustes diabatisation. These interpolated functions can be used in Quantics as follows. The examples are based on the QC DB generated from a short DD-vMCG simulation on butatriene. The DB contains the needed files
- database.sql SQLite database of ponts, energies, etc.
- but_fc_cas_freq.log QC calculation of frequencies at GS equilibrium geometry. Used to generate transformation between normal modes used in a simulation and the Cartesian coordinates of the QC DB.
- template.dat Template file used in DD simulation
- start.fchk File containing MOs at equilibrium geometry. Needed for the CASSCF QC calculations requested (using Gaussian)
Calculations proceed as a normal MCTDH calculation using the QC DB as a multidimensional potential. The input needs in addition the following keywords in the operator-section to point to the QC DB. At the moment the dynamics needs to be run in mass-frequency scaled normal mode coordinates.
| QC-DB Keywords in the OPERTAOR-SECTIOn | |
|---|---|
| File | Description |
| dbdata = S | S is a string with the directory name containg the QC-DB. This can be an absolute path or relative to the job directory |
| dbtransfile = S | S is a string with the name of the frequency file to set up the normal mode <-> Cartesian coordinate transformation. |
The operator is set up like a usual MCTDH operator file, defining the potentials and couplings using the potential name qchem{none}. See but_qcdb.op for an example. The coordinates are the usual mass-frequency scaled coordinates of the Vibronic Coupling Model, and the frequencies match those in the QC DB frequency file.
Types of Possible Calculations
- For a small system (2D-3D) the potentials can be put onto a grid for an exact WP propagation butdd_ex.inp
- For a mall system (2D-4D) the potentials can be put onto a grid for an MCTDH calculation using the multidimensional potential butdd_mctdh.inp
- For larger systems, the CDVR algorithm can be used in an MCTDH calculation butdd_cdvr.inp
- For medium sized systems, the POTFIT algorithm can be used to create the sum-of-products operator needed for an MCTDH calculation butdd_pfit.inp. See note below on creating Natpot files.
Using the POTFIT Program
It is possible to creat a sum-of-products form operator for an MCTDH calculation using the Potfit procedure and programs. Details of these programs are in the Quantics documentation. For converting a QC DB, each diabatic potential and coupling surface must be fit separately. Files for the butatriene example are given in butqcdb_pfit11.inp, butqcdb_pfit12.inp, and butqcdb_pfit22.inp. These inputs do not use any iterative improvements or other possible features of Potfit. The main keywords for using a QC DB mirror those for the Quantics program required to point to the location of the DB. It is also neecessary to define the state / coupling.| QC-DB Keywords in the OPERTAOR-SECTION of POTFIT | |
|---|---|
| File | Description |
| pes = qchem{none} | The surface to be fit is from a quantum chemistry calculation. |
| dbdata = S | S is a string with the directory name containg the QC-DB. This can be an absolute path or relative to the job directory |
| dbtransfile = S | S is a string with the name of the frequency file to set up the normal mode <-> Cartesian coordinate transformation. |
| dbstate = I, I1 | I and I1 are integers defining the potential surface (I=I1) or coupling (I /= I1) for the fit. |