modified on 23 February 2016 at 11:20 ••• 12,546 views


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NWChem Frequently Asked Questions


General information about NWChem

Where is the User's Manual?

NWChem User's Manual

Where do I go for help with a Global Arrays problem?

If you have problems with compiling the tools directory, please visit the Global Arrays Google group at or visit the Global Arrays website at

Where do I go for help with NWChem problems?

Please post your NWChem issue to the NWChem Community forums at

Where do I find the instructions for installing NWChem?

For updated instructions for compiling NWChem please visit the following URL

Installation Problems

How to fix configure: error: could not compile simple C MPI program

When compiling the tools directory, you might see the compilation stopping with the message configure: error: could not compile simple C MPI program This is most likely due to incorrect settings for the MPI_LIB, MPI_INCLUDE and LIBMPI environment variables. The suggested course of action is a) to use NWChem 6.6, b) unset all of the three variables above and c) point your PATH env. variable to the location of mpif90. If bash is your shell choice, this can be accomplished by typing

unset MPI_LIB
unset LIBMPI
export PATH="directory where mpif90 is located":$PATH

What's this business with ARMCI and ARMCI_NETWORK?

ARMCI is a library used by Global Arrays (both ARMCI and GA source code is located in NWChem's tools directory). More information can be found at the following URL If your installation uses a fast network and you are aiming to get optimal communication performance, you might want to assign a non-default value to ARMCI_NETWORK. The following links contained useful information about ARMCI_NETWORK: Choosing the ARMCI library and Setting up the proper environment_variables when compiling NWChem

Input Problems

I get the message: ! warning: processed input with no task. What is wrong?

Have you used emacs to create your input file? Emacs usually does not put and an end-of-line as a last character of the file, therefore the NWChem input parser ignores the last line of your input (the one containing the task directive). To fix the problem, add one more blank line after the task line and your task directive will be executed.

AUTOZ fails to generate valid internal coordinates. Now what?

If AUTOZ fails, NWChem will default to using Cartesian coordinates (and ignore any zcoord data) so you don't have to do anything unless you really need to use internal coordinates. An exception are certain cases where we have a molecule that contains a linear chain of 4 or more atoms, in which case the code will fail (see item 2. for work arounds). For small systems you can easily construct a Z-matrix, but for larger systems this can be quite hard.

First check your input. Are you using the correct units? The default is Angstroms. If you input atomic units but did not tell NWChem, then it's no wonder things are breaking. Also, is the geometry physically sensible? If atoms are too close to each other you'll get many unphysical bonds, whereas if they are too far apart AUTOZ will not be able to figure out how to connect things.

Once the obvious has been checked, there are several possible modes of failure, some of which may be worked around in the input.

1. Strictly linear molecules with 3 or more atoms. AUTOZ does not generate linear bend coordinates, but, just as in a real Z-matrix, you can specify a dummy center that is not co-linear. There are two relevant tips: i) constrain the dummy center to be not co-linear otherwise the center could become co-linear. Also, the inevitable small forces on the dummy center can confuse the optimizer. ii) put the dummy center far enough away so that only one connection is generated.

E.g., this input for acetylene will not use internals

             h  0  0  0
             c  0  0  1
             c  0  0  2.2
             h  0  0  3.2

           but this one will

               bond    2 3  3.0  cx constant
               angle 1 2 3 90.0 hcx constant
             h  0  0  0
             c  0  0  1
             x  3  0  1
             c  0  0  2.2
             h  0  0  3.2

2. Larger molecules that contain a strictly linear chain of four or more atoms (that ends in a free atom). For these molecules the autoz will fail and the code can currently not recover by using cartesians. One has to explicitly define noautoz in the geometry input to make it work. If internal coordinates are required one can fix it in the same manner as described above. However, you can also force a connection to a real nearby atom.

3. Very highly connected systems generate too many internal coordinates which can make optimization in redundant internals less efficient than in Cartesians. For systems such as clusters of atoms or small molecules, try using a smaller value of the scaling factor for covalent radii

zcoord; cvr_scaling 0.9; end

In addition to this you can also try specifying a minimal set of bonds to connect the fragments.

If these together don't work, then you're out of luck. Use Cartesians or construct a Z-matrix.

How do I restart a geometry optimization?

If you have saved the restart information that is kept in the permanent directory, then you can restart a calculation, as long as it did not crash while writing to the data base.

Following are two input files. The first starts a geometry optimization for ammonia. If this stops for nearly any reason such as it was interrupted, ran out of time or disk space, or exceeded the maximum number of iterations, then it may be restarted with the second job.

The key points are

  • The first job contains a START directive with a name for the calculation.
  • All subsequent jobs should contain a RESTART directive with the same name for the calculation.
  • All jobs must specify the same permanent directory. The default permanent directory is the current directory.
  • If you want to change anything in the restart job, just put the data before the task directive. Otherwise, all options will be the same as in the original job.

Job 1.

         start ammonia
         permanent_dir /u/myfiles

             h 1 nh
             h 1 nh 2 hnh
             h 1 nh 2 hnh 3 hnh -1
             nh 1.
             hnh 115.

           n library 3-21g; h library 3-21g

         task scf optimize

Job 2.

         restart ammonia
         permanent_dir /u/myfiles

         task scf optimize

Execution Problems

How do I set the correct value for ARMCI_DEFAULT_SHMMAX?

Some ARMCI_NETWORK values (e.g. OPENIB) depend on the ARMCI_DEFAULT_SHMMAX value for large allocations of Global memory. We recommend a value of -- at least -- 2048, e.g. in bash shell parlance


A value of 2048 for ARMCI_DEFAULT_SHMMAX corresponds to 2048 GBytes, equal to 2048*1024*1024=2147483648 bytes. For ARMCI_DEFAULT_SHMMAX=2048 to work, it is necessary that kernel parameter kernel.shmmax to be greater than 2147483648. You can check the current value of kernel.shmmax on your system by typing

sysctl kernel.shmmax

More detail about kernel.shmmax can be found at the webpage