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2.2 Running Q-Chem

2.2.2 Integration with IQmol

(May 7, 2024) Installation and Server Setup

IQmol provides a fully integrated molecular builder and viewer for the Q-Chem package. It is available for the Windows, Linux, and Mac OS X platforms and instructions for downloading and installing the latest version can be found at www.iqmol.org/downloads.html.

IQmol can be run as a stand-alone package which is able to open existing Q-Chem input/output files, but it can also be used as a fully functional front end which is able to submit and monitor Q-Chem jobs, and to analyze the resulting output. By default, IQmol submits Q-Chem jobs to a server that is owned by Q-Chem, Inc., which provides prospective users with the opportunity to run short Q-Chem demonstration jobs for free simply by downloading IQmol, without the need to install Q-Chem.

For customers who own Q-Chem, it is necessary to configure IQmol to submit jobs to an appropriate server. To do this, first ensure Q-Chem has been correctly installed on the target machine and can be run from the command line. Second, open IQmol and carry out the following steps:

  1. 1.

    Select the CalculationEdit Servers menu option. A dialog will appear with a list of configured servers (which will initially be empty).

  2. 2.

    Click the Add New Server button with the ‘+’ icon. This opens a dialog which allows the new server to be configured. The server is the machine which has your Q-Chem installation.

  3. 3.

    Give the server a name (this is simply used to identify the current server configuration and does not have to match the actual machine name) and select if the machine is local (i.e. the same machine as IQmol is running on) or remote.

  4. 4.

    If there is PBS software running on the server, select the PBS ‘Type’ option, otherwise in most cases the Basic option should be sufficient. Please note that the server must be Linux based and cannot be a Windows server.

  5. 5.

    If required, the server can be further configured using the Configure button. Details on this can be found in the embedded IQmol help which can be accessed via the HelpShow Help menu option.

  6. 6.

    For non-PBS servers the number of concurrent Q-Chem jobs can be limited using a simple inbuilt queuing system. The maximum number of jobs is set by the Job Limit control. If the Job Limit is set to zero the queue is disabled and any number of jobs can be run concurrently. Please note that this limit applies to the current IQmol session and does not account for jobs submitted by other users or by other IQmol sessions.

  7. 7.

    The $QC environment variable should be entered in the given box.

  8. 8.

    For remote servers the address of the machine and your user name are also required. IQmol uses SSH2 to connect to remote machines and the most convenient way to set this up is by using authorized keys () for details on how these can be set up). IQmol can then connect via the SSH Agent and will not have to prompt you for your password. If you are not able to use an SSH Agent, several other authentication methods are offered:

    • Public Key This requires you to enter your SSH passphrase (if any) to unlock your private key file. The passphrase is stored in memory, not disk, so you will need to re-enter this each time IQmol is run.

    • Password Prompt This requires each server password to be entered each time IQmol is run. Once the connection has been established the memory used to hold the password is overwritten to reduce the risk of recovery from a core dump.

    Further configuration of SSH options should not be required unless your public/private keys are stored in a non-standard location.

It is recommended that you test the server configuration to ensure everything is working before attempting to submit a job. Multiple servers can be configured if you have access to more than one copy of Q-Chem or have different account configurations. In this case the default server is the first on the list and if you want to change this you should use the arrow buttons in the Server List dialog. The list of configured servers will be displayed when submitting Q-Chem jobs and you will be able to select the desired server for each job.

Please note that while Q-Chem is file-based, as of version 2.1 IQmol uses a directory to keep the various files from a calculation. More details can be found in the IQmol user manual. Reading Q-Chem Outputs

In addition to having IQmol communicate with Q-Chem, it can work standalone by reading output files directly. A number of file formats are supported:

  • Q-Chem outputs produced by running the qchem shell script (Section 2.2.1)

  • Formatted checkpoint (.fchk) files: A formatted checkpoint file can be requested by setting GUI = 2 in the $rem section of the input, or equivalently by setting IQMOL_FCHK = TRUE. The checkpoint file name is determined by the $GUIFILE environment variable which by default is set to ${input}.fchk.

  • qarchive files: We have created a new file format designed to overcome limitations of using text-based formats with post-processing and visualization tools. The qarchive format is HDF5-based, supports more of Q-Chem’s novel feature set than fchk files, and is tightly integrated with IQmol. In order to create this file in your working directory, pass the -archive flag to the qchem shell script and add the following to the input:

            enable_archive = True !Turns on generation of Archive

    The archive file is also always present in each job’s scratch directory when the -save flag is used. It appears under the file name $QCSCRATCH/savename/qarchive.h5