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Q-Chem

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General Description

Q-Chem is a comprehensive ab initio quantum chemistry package for accurate predictions of molecular structures, reactivities, and vibrational, electronic and NMR spectra. Q-Chem represents the state-of-the-art of methodology from the highest performance DFT/HF calculations to high level post-HF correlation methods.

Parallel Q-Chem is capable of carrying out energy (HF & DFT), gradient (HF & DFT), and second derivative (HF, DFT) calculations over multiple shared and distributed processors. Parallel second derivative calculations at MP2 and OD levels with finite difference method are also possible. Other types of calculations can be done only on a single processor. Using Parallel Q-Chem on a single processor runs exactly same as the serial version.

Basic Usage

A basic serial Q-Chem submission script is shown below:

#!/bin/csh
#$ -M <your_netid>@nd.edu
#$ -pe smp 1
#$ -m abe
#$ -q debug  #"debug" for job runs fewer than 4hours, "long" for longer than 4 hours, killed at 15 days.

module load qchem

qchem -seq script.input script.output

for the qchem calculations which support parallel computation, the OpenMPI Message Passing environment is be used. An example parallel script is shown below,

#!/bin/csh
#$ -M <your_netid>@nd.edu
#$ -pe mpi-12 24
#$ -m abe
#$ -N  <your_jobname> 
module load qchem

qchem -par -np $NSLOTS script.input script.output

Useful Options

Q-CHEM uses a large amount of temporary scratch files when performing calculations. The directory where these scratch files are located is set by the QSCRATCH environment variable. By default the Q-CHEM module sets QSCRATCH to $HOME/qchem.

Users running many Q-Chem calculations may wish to allocate a large amount of space for this purpose; for instance, a directory in their /scratch365 area. This can be done in the script, after the qchem module has been loaded, with the commands

setenv QCSCRATCH /scratch365/your_netID/qchem

or

export QCSCRATCH=/scratch365/your_netID/qchem

for the csh or bash shells, respectively.

Additional Information

For BSSE (Basis Sets Superposition Error) calculations, the basis set keyword has to be declared as 'mixed' and $basis section has to be included.

Further Information

See the official website: Q-Chem

User Manual: Q-Chem Manual 4.0