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Getting Started

This section provides an overview of NWChem input and program architecture, and the syntax used to describe the input. See Simple Input File and Water Molecule Input for examples of NWChem input files with detailed explanation.

NWChem consists of independent modules that perform the various functions of the code. Examples of modules include the input parser, SCF energy, SCF analytic gradient, DFT energy, etc.. Data is passed between modules and saved for restart using a disk-resident database or dumpfile (see NWChem Architecture).

The input to NWChem is composed of commands, called directives, which define data (such as basis sets, geometries, and filenames) and the actions to be performed on that data. Directives are processed in the order presented in the input file, with the exception of certain start-up directives (see Input File Structure) which provide critical job control information, and are processed before all other input. Most directives are specific to a particular module and define data that is used by that module only. A few directives (see Top-level Directives) potentially affect all modules, for instance by specifying the total electric charge on the system.

There are two types of directives. Simple directives consist of one line of input, which may contain multiple fields. Compound directives group together multiple simple directives that are in some way related and are terminated with an END directive. See the sample inputs (Simple Input File and Water Molecule Input) and the input syntax specification (Input Format and Syntax for Directives).

All input is free format and case is ignored except for actual data (e.g., names/tags of centers, titles). Directives or blocks of module-specific directives (i.e., compound directives) can appear in any order, with the exception of the TASK directive (see Input File Structure and Tasks) which is used to invoke an NWChem module. All input for a given task must precede the TASK directive. This input specification rule allows the concatenation of multiple tasks in a single NWChem input file.

To make the input as short and simple as possible, most options have default values. The user needs to supply input only for those items that have no defaults, or for items that must be different from the defaults for the particular application. In the discussion of each directive, the defaults are noted, where applicable.

The input file structure is described in the following sections, and illustrated with two examples. The input format and syntax for directives is also described in detail.

.nwchemrc for environment variables and libraries

Each user should have a .nwchemrc file to point to default data files, such as basis sets, pseudopotentials, and MD potentials.

Contents of the default.nwchemrc file based on the above information should be:

 nwchem_basis_library <location of NWChem installation>/src/basis/libraries/  
 nwchem_nwpw_library <location of NWChem installation>/src/nwpw/libraryps/  
 ffield amber  
 amber_1 <location of NWChem installation>/src/data/amber_s/  
 amber_2 <location of NWChem installation>/src/data/amber_q/  
 amber_3 <location of NWChem installation>/src/data/amber_x/  
 amber_4 <location of NWChem installation>/src/data/amber_u/  
 spce   <location of NWChem installation>/src/data/solvents/spce.rst  
 charmm_s <location of NWChem installation>/src/data/charmm_s/  
 charmm_x <location of NWChem installation>/src/data/charmm_x/

It is can also be useful to use the NWCHEM_BASIS_LIBRARY environment variable when testing a new libraries in your own directory. This will allow you to overwrite the value of nwchem_basis_library in your .nwchemrc file and point to the new basis library. For example:

   % setenv NWCHEM_BASIS_LIBRARY "$NWCHEM/data-5.0/libraries/"

Do not forget the trailing "/".

Input File Structure

The structure of an input file reflects the internal structure of NWChem. At the beginning of a calculation, NWChem needs to determine how much memory to use, the name of the database, whether it is a new or restarted job, where to put scratch/permanent files, etc.. It is not necessary to put this information at the top of the input file, however. NWChem will read through the entire input file looking for the start-up directives. In this first pass, all other directives are ignored.

The start-up directives are


After the input file has been scanned for the start-up directives, it is rewound and read sequentially. Input is processed either by the top-level parser (for the directives listed in Top-level Directives, such as TITLE, SET, …) or by the parsers for specific computational modules (e.g., SCF, DFT, …). Any directives that have already been processed (e.g., MEMORY) are ignored. Input is read until a TASK directive (see Tasks) is encountered. A TASK directive requests that a calculation be performed and specifies the level of theory and the operation to be performed. Input processing then stops and the specified task is executed. The position of the TASK directive in effect marks the end of the input for that task. Processing of the input resumes upon the successful completion of the task, and the results of that task are available to subsequent tasks in the same input file.

The name of the input file is usually provided as an argument to the execute command for NWChem. That is, the execute command looks something like the following

 nwchem input_file

The default name for the input file is nwchem.nw. If an input file name input_file is specified without an extension, the code assumes .nw as a default extension, and the input filename becomes input_file.nw. If the code cannot locate a file named either input_file or input_file.nw (or nwchem.nw if no file name is provided), an error is reported and execution terminates. The following section presents two input files to illustrate the directive syntax and input file format for NWChem applications.

Simple Input File – SCF geometry optimization

A simple example of an NWChem input file is an SCF geometry optimization of the nitrogen molecule, using a Dunning cc-pvdz basis set. This input file contains the bare minimum of information the user must specify to run this type of problem – fewer than ten lines of input, as follows:

 title "Nitrogen cc-pvdz SCF geometry optimization"  
   n 0 0 0  
   n 0 0 1.08  
   n library cc-pvdz  
 task scf optimize

Examining the input line by line, it can be seen that it contains only four directives; TITLE, GEOMETRY, BASIS, and TASK. The TITLE directive is optional, and is provided as a means for the user to more easily identify outputs from different jobs. An initial geometry is specified in Cartesian coordinates and Angstrøms by means of the GEOMETRY directive. The Dunning cc-pvdz basis is obtained from the NWChem basis library, as specified by the BASIS directive input. The TASK directive requests an SCF geometry optimization.

The GEOMETRY directive defaults to Cartesian coordinates and Angstrøms (options include atomic units and Z-matrix format). The input blocks for the BASIS and GEOMETRY directives are structured in similar fashion, i.e., name, keyword, …, end (In this simple example, there are no keywords). The BASIS input block must contain basis set information for every atom type in the geometry with which it will be used. Refer to Basis for a description of available basis sets and a discussion of how to define new ones.

The last line of this sample input file (task scf optimize) tells the program to optimize the molecular geometry by minimizing the SCF energy. (For a description of possible tasks and the format of the TASK directive, refer to Tasks)

If the input is stored in the file n2.nw, the command to run this job on a typical UNIX workstation is as follows:

 nwchem n2

NWChem output is to UNIX standard output, and error messages are sent to both standard output and standard error.

Water Molecule Sample Input File

A more complex sample problem is the optimization of a positively charged water molecule using second-order Møller-Plesset perturbation theory (MP2), followed by a computation of frequencies at the optimized geometry. A preliminary SCF geometry optimization is performed using a computationally inexpensive basis set (STO-3G). This yields a good starting guess for the optimal geometry, and any Hessian information generated will be used in the next optimization step. Then the optimization is finished using MP2 and a basis set with polarization functions. The final task is to calculate the MP2 vibrational frequencies. The input file to accomplish these three tasks is as follows:

start h2o_freq  
charge 1  
geometry units angstroms  
 O       0.0  0.0  0.0  
 H       0.0  0.0  1.0  
 H       0.0  1.0  0.0  
 H library sto-3g  
 O library sto-3g  
 uhf; doublet  
 print low  
title "H2O+ : STO-3G UHF geometry optimization"  
task scf optimize  
 H library 6-31g**  
 O library 6-31g**  
title "H2O+ : 6-31g** UMP2 geometry optimization"  
task mp2 optimize  
mp2; print none; end  
scf; print none; end  
title "H2O+ : 6-31g** UMP2 frequencies"  
task mp2 freq

The START directive (START/RESTART tells NWChem that this run is to be started from the beginning. This directive need not be at the beginning of the input file, but it is commonly placed there. Existing database or vector files are to be ignored or overwritten. The entry h2o_freq on the START line is the prefix to be used for all files created by the calculation. This convention allows different jobs to run in the same directory or to share the same scratch directory SCRATCH_DIR/PERMANENT_DIR, as long as they use different prefix names in this field.

As in the first sample problem, the geometry is given in Cartesian coordinates. In this case, the units are specified as Angstrøms. (Since this is the default, explicit specification of the units is not actually necessary, however.) The CHARGE directive defines the total charge of the system. This calculation is to be done on an ion with charge +1.

A small basis set (STO-3G) is specified for the intial geometry optimization. Next, the multiple lines of the first SCF directive in the scf …end block specify details about the SCF calculation to be performed. Unrestricted Hartree-Fock is chosen here (by specifying the keyword uhf), rather than the default, restricted open-shell high-spin Hartree-Fock (ROHF). This is necessary for the subsequent MP2 calculation, because only UMP2 is currently available for open-shell systems (see Section 4). For open-shell systems, the spin multiplicity has to be specified (using doublet in this case), or it defaults to singlet. The print level is set to low to avoid verbose output for the starting basis calculations.

All input up to this point affects only the settings in the runtime database. The program takes its information from this database, so the sequence of directives up to the first TASK directive is irrelevant. An exchange of order of the different blocks or directives would not affect the result. The TASK directive, however, must be specified after all relevant input for a given problem. The TASK directive causes the code to perform the specified calculation using the parameters set in the preceding directives. In this case, the first task is an SCF calculation with geometry optimization, specified with the input scf and optimize. (See Tasks for a list of available tasks and operations.)

After the completion of any task, settings in the database are used in subsequent tasks without change, unless they are overridden by new input directives. In this example, before the second task (task mp2 optimize), a better basis set (6-31G**) is defined and the title is changed. The second TASK directive invokes an MP2 geometry optimization.

Once the MP2 optimization is completed, the geometry obtained in the calculation is used to perform a frequency calculation. This task is invoked by the keyword freq in the final TASK directive, task mp2 freq. The second derivatives of the energy are calculated as numerical derivatives of analytical gradients. The intermediate energies and gradients are not of interest in this case, so output from the SCF and MP2 modules is disabled with the PRINT directives.

Input Format and Syntax for Directives

This section describes the input format and the syntax used in the rest of this documentation to describe the format of directives. The input format for the directives used in NWChem is similar to that of UNIX shells, which is also used in other chemistry packages, most notably GAMESS-UK. An input line is parsed into whitespace (blanks or tabs) separating tokens or fields. Any token that contains whitespace must be enclosed in double quotes in order to be processed correctly. For example, the basis set with the descriptive name modified Dunning DZ must appear in a directive as “modified Dunning DZ”, since the name consists of three separate words.

Input Format

A (physical) line in the input file is terminated with a newline character (also known as a `return’ or `enter’ character). A semicolon (;) can be also used to indicate the end of an input line, allowing a single physical line of input to contain multiple logical lines of input. For example, five lines of input for the GEOMETRY directive can be entered as follows;

  O 0  0     0  
  H 0  1.430 1.107  
  H 0 -1.430 1.107  

These same five lines could be entered on a single line, as

 geometry; O 0 0 0; H 0 1.430 1.107; H 0 -1.430 1.107; end

This one physical input line comprises five logical input lines. Each logical or physical input line must be no longer than 1023 characters.

In the input file:

  • a string, token, or field is a sequence of ASCII characters (NOTE: if the string includes blanks or tabs (i.e., white space), the entire string must be enclosed in double quotes).
  • \ (backslash) at the end of a line concatenates it with the next line. Note that a space character is automatically inserted at this point so that it is not possible to split tokens across lines. A backslash is also used to quote special characters such as whitespace, semi-colons, and hash symbols so as to avoid their special meaning (NOTE: these special symbols must be quoted with the backslash even when enclosed within double quotes).
  • ; (semicolon) is used to mark the end of a logical input line within a physical line of input.
  • # (the hash or pound symbol) is the comment character. All characters following # (up to the end of the physical line) are ignored.
  • If any input line (excluding Python programs begins with the string INCLUDE (ignoring case) and is followed by a valid file name, then the data in that file are read as if they were included into the current input file at the current line. Up to three levels of nested include files are supported. The user should note that inputting a basis set from the standard basis library (Basis Sets) uses one level of include.
  • Data is read from the input file until an end-of-file is detected, or until the string EOF (ignoring case) is encountered at the beginning of an input line.

Format and syntax of directives

Directives consist of a directive name, keywords, and optional input, and may contain one line or many. Simple directives consist of a single line of input with one or more fields. Compound directives can have multiple input lines, and can also include other optional simple and compound directives. A compound directive is terminated with an END directive. The directives START (see START/RESTART) and ECHO (see ECHO) are examples of simple directives. The directive GEOMETRY (see Geometry) is an example of a compound directive.

Some limited checking of the input for self-consistency is performed by the input module, but most defaults are imposed by the application modules at runtime. It is therefore usually impossible to determine beforehand whether or not all selected options are consistent with each other.

In the rest of this document, the following notation and syntax conventions are used in the generic descriptions of the NWChem input.

  • a directive name always appears in all-capitals, and in computer typeface (e.g., GEOMETRY, BASIS, SCF). Note that the case of directives and keywords is ignored in the actual input.
  • a keyword always appears in lower case, in computer typeface (e.g., swap, print, units, bqbq).
  • variable names always appear in lower case, in computer typeface, and enclosed in angle brackets to distinguish them from keywords (e.g., <input_filename>, <basisname>, <tag>).
  • $variable$ is used to indicate the substitution of the value of a variable.
  • () is used to group items (the parentheses and other special symbols should not appear in the input).
  • || separate exclusive options, parameters, or formats.
  • [ ] enclose optional entries that have a default value.
  • < > enclose a type, a name of a value to be specified, or a default value, if any.
  • \ is used to concatenate lines in a description.
  • ... is used to indicate indefinite continuation of a list.

An input parameter is identified in the description of the directive by prefacing the name of the item with the type of data expected, i.e.,

  • string - an ASCII character string
  • integer - integer value(s) for a variable or an array
  • logical - true/false logical variable
  • real - real floating point value(s) for a variable or an array
  • double - synonymous with real

If an input item is not prefaced by one of these type names, it is assumed to be of type ``string’‘.

In addition, integer lists may be specified using Fortran triplet notation, which interprets lo:hi:inc as lo, lo+inc, lo+2*inc, …, hi. For example, where a list of integers is expected in the input, the following two lines are equivalent

  7 10 21:27:2 1:3 99  
  7 10 21 23 25 27 1 2 3 99

(In Fortran triplet notation, the increment, if unstated, is 1; e.g., 1:3 = 1:3:1.)

The directive VECTORS is presented here as an example of an NWChem input directive. The general form of the directive is as follows:

 VECTORS [input (<string input_movecs default atomic>) || \  
                  (project <string basisname> <string filename>)] \ 
         [swap [(alpha||beta)] <integer vec1 vec2> ...] \ 
         [output <string output_movecs default $file_prefix$.movecs>]

This directive contains three optional keywords, as indicated by the three main sets of square brackets enclosing the keywords input, swap, and output. The keyword input allows the user to specify the source of the molecular orbital vectors. There are two mutually exclusive options for specifying the vectors, as indicated by the || symbol separating the option descriptions;

 (<string input_movecs default atomic>) || \ 
                 (project <string basisname> <string filename>) \

The first option, <string input_movecs default atomic>, can be used to specify an ASCII character string for the parameter input_movecs. If no entry is specified, the code uses the default atomic (i.e., atomic guess). The second option, project <string basisname> <string filename>, contains the keyword project, which takes two string arguments. When this keyword is used, the vectors in file <filename> will be projected from the (smaller) basis <basisname> into the current atomic orbital (AO) basis.

The second keyword, swap, can be used to re-order the starting vectors, specifying the pairs of vectors to be swapped. As many pairs as the user wishes to have swapped can be listed for . The optional keywords alpha and beta allow the user to swap the alpha or beta spin orbitals.

The third keyword, output, allows the user to tell the code where to store the vectors, by specifying an ASCII string for the parameter output_movecs. If no entry is specified for this parameter, the default is to write the vectors back into either the user- specified MO vectors input file or, if this is not available, the file $file_prefix$.movecs.

A particular example of the VECTORS directive is shown below. It specifies both the input and output keywords, but does not use the swap option.

 vectors input project "small basis" small_basis.movecs \
         output large_basis.movecs

This directive tells the code to generate input vectors by projecting from vectors in a smaller basis named “small basis”, which is stored in the file small_basis.movecs. The output vectors will be stored in the file large_basis.movecs.

The order of keyed optional entries within a directive should not matter, unless noted otherwise in the specific instructions for a particular directive.