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Start Restart

START / RESTART

The START or RESTART directive define the start-up mode and are optional keywords. If one of these two directives is not specified explicitly, the code will infer one, based upon the name of the input file and the availability of the database. When allowing NWChem to infer the start-up directive, the user must be quite certain that the contents of the database will result in the desired action. It is usually more prudent to specify the directive explicitly, using the following format:

(RESTART || START) [<string file_prefix default input_file_prefix>] \  
                   [rtdb <string rtdb_file_name default file_prefix.db>]

The START directive indicates that the calculation is one in which a new database is to be created. Any relevant information that already exists in a previous database of the same name is destroyed. The string variable file_prefix will be used as the prefix to name any files created in the course of the calculation.

E.g., to start a new calculation on water, one might specify

start water

which will make all files begin with “water.”.

If the user does not specify an entry for file_prefix on the START directive (or omits the START directive altogether), the code uses the base-name of the input file as the file prefix. That is, the variable file_prefix is assigned the name of the input file (not its full pathname), but without the last “dot-suffix”. For example, the input file name /home/dave/job.2.nw yields job.2 as the file prefix, if a name is not assigned explicitly using the START directive.

The user also has the option of specifying a unique name for the database, using the keyword rtdb. When this keyword is entered, the string entered for rtdb_file_name is used as the database name. If the keyword rtdb is omitted, the name of the database defaults to file_prefix.db in the directory for permanent files.

If a calculation is to start from a previous calculation and go on using the existing database, the RESTART directive must be used. In such a case, the previous database must already exist. The name specified for file_prefix usually should not be changed when restarting a calculation. If it is changed, NWChem will not be able to find needed files when going on with the calculation.

In the most common situation, the previous calculation was completed (with or without an error condition), and it is desired to perform a new task or restart the previous one, perhaps with some input changes. In these instances, the RESTART directive should be used. This re-uses the previous database and associated files, and reads the input file for new input and task information.

The RESTART directive looks immediately for new input and task information, deleting information about previous incomplete tasks. For example, when doing a RESTART there is no need to specify geometry or basis set declaration because the program will detect this information since it is stored in the run-time database.

If a calculation runs out of time, for example because it is on a queuing system, this is another instance where doing a RESTART is advisable. Simply include nothing after the RESTART directive except those tasks that are unfinished.

To summarize the default options for this start-up directive, if the input file does not contain a START or a RESTART directive, then

  • the variable file_prefix is assigned the name of the input file for the job, without the suffix (which is usually .nw)
  • the variable rtdb_file_name is assigned the default name, file_prefix.db

If the database with name file_prefix.db does not already exist, the calculation is carried out as if a START directive had been encountered. If the database with name file_prefix.db does exist, then the calculation is performed as if a RESTART directive had been encountered.

For example, NWChem can be run using an input file with the name water.nw by typing the UNIX command line,

nwchem water.nw

If the NWChem input file water.nw does not contain a START or RESTART directive, the code sets the variable file_prefix to water. Files created by the job will have this prefix, and the database will be named water.db. If the database water.db does not exist already, the code behaves as if the input file contains the directive,

start water

If the database water.db does exist, the code behaves as if the input file contained the directive,

restart water

Use of permanent_dir

We suggest the user to add the permanent directory line to the input file. This allows to store files in a specific directory for easy re-use between start and restart stages.
The start file then becomes

start water
permanent_dir /home/doe/nwchem_files

while the restart file becomes

restart water
permanent_dir /home/doe/nwchem_files