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# Release65:Python

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# Controlling NWChem with Python

Python (version 1.5.1) programs may be embedded into the NWChem input and used to control the execution of NWChem. Python is a very powerful and widely used scripting language that provides useful things such as variables, conditional branches and loops, and is also readily extended. Example applications include scanning potential energy surfaces, computing properties in a variety of basis sets, optimizing the energy w.r.t. parameters in the basis set, computing polarizabilities with finite field, and simple molecular dynamics.

Look in the NWChem contrib directory for useful scripts and examples. Visit the Python web-site http://www.python.org for a full manual and lots of useful code and resources.

## How to input and run a Python program inside NWChem

A Python program is input into NWChem inside a Python compound directive.

 python [print|noprint]
...
end


The END directive must be flush against the left margin (see the Troubleshooting section for the reason why).

The program is by default printed to standard output when read, but this may be disabled with the noprint keyword. Python uses indentation to indicate scope (and the initial level of indentation must be zero), whereas NWChem uses optional indentation only to make the input more readable. For example, in Python, the contents of a loop, or conditionally-executed block of code must be indented further than the surrounding code. Also, Python attaches special meaning to several symbols also used by NWChem. For these reasons, the input inside a PYTHON compound directive is read verbatim except that if the first line of the Python program is indented, the same amount of indentation is removed from all subsequent lines. This is so that a program may be indented inside the PYTHON input block for improved readability of the NWChem input, while satisfying the constraint that when given to Python the first line has zero indentation.

E.g., the following two sets of input specify the same Python program.

 python
print 'Hello'
print 'Goodbye'
end

 python
print 'Hello'
print 'Goodbye'
end


whereas this program is in error since the indentation of the second line is less than that of the first.

 python
print 'Hello'
print 'Goodbye'
end


The Python program is not executed until the following directive is encountered

 task python


which is to maintain consistency with the behavior of NWChem in general. The program is executed by all nodes. This enables the full functionality and speed of NWChem to be accessible from Python, but there are some gotchas

• Print statements and other output will be executed by all nodes so you will get a lot more output than probably desired unless the output is restricted to just one node (by convention node zero).
• The calls to NWChem functions are all collective (i.e., all nodes must execute them). If these calls are not made collectively your program may deadlock (i.e., cease to make progress).
• When writing to the database (rtdb_put()) it is the data from node zero that is written.
• NWChem overrides certain default signal handlers so care must be taken when creating processes (see Section 38.3.11).

## NWChem extensions

Since we have little experience using Python, the NWChem-Python interface might change in a non-backwardly compatible fashion as we discover better ways of providing useful functionality. We would appreciate suggestions about useful things that can be added to the NWChem-Python interface. In principle, nearly any Fortran or C routine within NWChem can be extended to Python, but we are also interested in ideas that will enable users to build completely new things. For instance, how about being able to define your own energy functions that can be used with the existing optimizers or dynamics package?

Python has been extended with a module named "nwchem" which is automatically imported and contains the following NWChem-specific commands. They all handle NWChem-related errors by raising the exception "NWChemError", which may be handled in the standard Python manner.

• input_parse(string) -- invokes the standard NWChem input parser with the data in string as input. Note that the usual behavior of NWChem will apply -- the parser only reads input up to either end of input or until a TASK directive is encountered (the task directive is not executed by the parser).
• task_energy(theory) -- returns the energy as if computed with the NWChem directive TASK ENERGY <THEORY>.
• task_optimize(theory) -- returns a tuple (energy,gradient) as if computed with the NWChem directive TASK OPTIMIZE <THEORY>. The energy and gradient will be those at the last point in the optimization and consistent with the current geometry in the database.
• ga_nodeid() -- returns the number of the parallel process.
• rtdb_print(print_values) -- prints the contents of the RTDB. If print_values is 0, only the keys are printed, if it is 1 then the values are also printed.
• rtdb_put(name, values) or rtdb_put(name, values, type) -- puts the values into the database with the given name. In the first form, the type is inferred from the first value, and in the second form the type is specified using the last argument as one of INT, DBL, LOGICAL, or CHAR.
• rtdb_get(name) -- returns the data from the database associated with the given name.

An example below explains, in lieu of a Python wrapper for the geometry object, how to obtain the Cartesian molecular coordinates directly from the database.

## Examples

Several examples will provide the best explanation of how the extensions are used, and how Python might prove useful.

### Hello world

 python
print 'Hello world from process ', ga_nodeid()
end



This input prints the traditional greeting from each parallel process.

### Scanning a basis exponent

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

python
exponent = 0.1
while (exponent <= 2.01):
input_parse(
basis noprint
H library 3-21g; O library 3-21g; O d; %f 1.0
end
% (exponent))
print ' exponent = ', exponent, ' energy = ', task_energy('scf')
exponent = exponent + 0.1
end

print none



This program augments a 3-21g basis for water with a d-function on oxygen and varies the exponent from 0.1 to 2.0 in steps of 0.1, printing the exponent and energy at each step.

The geometry is input as usual, but the basis set input is embedded inside a call to input_parse() in the Python program. The standard Python string substitution is used to put the current value of the exponent into the basis set (replacing the %f) before being parsed by NWChem. The energy is returned by task_energy('scf') and printed out. The print none in the NWChem input switches off all NWChem output so all you will see is the output from your Python program.

Note that execution in parallel may produce unwanted output since all process execute the print statement inside the Python program.

Look in the NWChem contrib directory for a routine that makes the above task easier.

### Scanning a basis exponent revisited

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

print none

python
if (ga_nodeid() == 0): plotdata = open("plotdata",'w')

def energy_at_exponent(exponent):
input_parse('''
basis noprint
H library 3-21g; O library 3-21g; O d; %f 1.0
end
''' % (exponent))

exponent = 0.1
while exponent <= 2.01:
energy = energy_at_exponent(exponent)
if (ga_nodeid() == 0):
print ' exponent = ', exponent, ' energy = ', energy
plotdata.write('%f %f\n' % (exponent , energy))
exponent = exponent + 0.1

if (ga_nodeid() == 0): plotdata.close()
end



This input performs exactly the same calculation as the previous one, but uses a slightly more sophisticated Python program, also writes the data out to a file for easy visualization with a package such as gnuplot, and protects write statements to prevent duplicate output in a parallel job. The only significant differences are in the Python program. A file called "plotdata" is opened, and then a procedure is defined which given an exponent returns the energy. Next comes the main loop that scans the exponent through the desired range and prints the results to standard output and to the file. When the loop is finished the additional output file is closed.

### Scanning a geometric variable

python
geometry = '''
geometry noprint; symmetry d2h
C 0 0 %f; H 0  0.916 1.224
end
'''
x = 0.6
while (x < 0.721):
input_parse(geometry % x)
print ' x = %5.2f   energy = %10.6f' % (x, energy)
x = x + 0.01
end

basis; C library 6-31g*; H library 6-31g*; end

print none



This scans the bond length in ethene from 1.2 to 1.44 in steps of 0.2 computing the energy at each geometry. Since it is using D2h symmetry the program actually uses a variable (x) that is half the bond length.

Look in the NWChem contrib directory for a routine that makes the above task easier.

### Scan using the BSSE counterpoise corrected energy

basis spherical
Ne library cc-pvdz; BqNe library Ne cc-pvdz
He library cc-pvdz; BqHe library He cc-pvdz
end

mp2; tight; freeze core atomic; end

print none

python noprint
supermolecule = 'geometry noprint;   Ne 0 0 0;   He 0 0 %f; end\n'
fragment1     = 'geometry noprint;   Ne 0 0 0; BqHe 0 0 %f; end\n'
fragment2     = 'geometry noprint; BqNe 0 0 0;   He 0 0 %f; end\n'

def energy(geometry):
input_parse(geometry + 'scf; vectors atomic; end\n')

def bsse_energy(z):
return energy(supermolecule % z) - \
energy(fragment1 % z) - \
energy(fragment2 % z)
z = 3.3
while (z < 4.301):
e = bsse_energy(z)
if (ga_nodeid() == 0):
print ' z = %5.2f   energy = %10.7f ' % (z, e)
z = z + 0.1
end



This example scans the He--Ne bond-length from 3.3 to 4.3 and prints out the BSSE counterpoise corrected MP2 energy.

The basis set is specified as usual, noting that we will need functions on ghost centers to do the counterpoise correction. The Python program commences by defining strings containing the geometry of the super-molecule and two fragments, each having one variable to be substituted. Next, a function is defined to compute the energy given a geometry, and then a function is defined to compute the counterpoise corrected energy at a given bond length. Finally, the bond length is scanned and the energy printed. When computing the energy, the atomic guess has to be forced in the SCF since by default it will attempt to use orbitals from the previous calculation which is not appropriate here.

Since the counterpoise corrected energy is a linear combination of other standard energies, it is possible to compute the analytic derivatives term by term. Thus, combining this example and the next could yield the foundation of a BSSE corrected geometry optimization package.

### Scan the geometry and compute the energy and gradient

basis noprint; H library sto-3g; O library sto-3g; end

python noprint
print '   y     z     energy                gradient'
print ' ----- ----- ---------- ------------------------------------'
y = 1.2
while y <= 1.61:
z = 1.0
while z <= 1.21:
input_parse('''
geometry noprint units atomic
O 0   0   0
H 0  %f -%f
H 0 -%f -%f
end
''' % (y, z, y, z))

print ' %5.2f %5.2f %9.6f' % (y, z, energy),
i = 0
i = i + 1
print ''
z = z + 0.1
y = y + 0.1
end

print none



This program illustrates evaluating the energy and gradient by calling task_gradient(). A water molecule is scanned through several C2v geometries by varying the y and z coordinates of the two hydrogen atoms. At each geometry the coordinates, energy and gradient are printed.

The basis set (sto-3g) is input as usual. The two while loops vary the y and z coordinates. These are then substituted into a geometry which is parsed by NWChem using input_parse(). The energy and gradient are then evaluated by calling task_gradient() which returns a tuple containing the energy (a scalar) and the gradient (a vector or list). These are printed out exploiting the Python convention that a print statement ending in a comma does not print end-of-line.

### Reaction energies varying the basis set

mp2; freeze atomic; end

print none

python
energies = {}
c2h4 = 'geometry noprint; symmetry d2h; \
C 0 0 0.672; H 0 0.935 1.238; end\n'
ch4  = 'geometry noprint; symmetry td; \
C 0 0 0; H 0.634 0.634 0.634; end\n'
h2   = 'geometry noprint; H 0 0 0.378; H 0 0 -0.378; end\n'

def energy(basis, geometry):
input_parse('''
basis spherical noprint
c library %s ; h library %s
end
''' % (basis, basis))
input_parse(geometry)

for basis in ('sto-3g', '6-31g', '6-31g*', 'cc-pvdz', 'cc-pvtz'):
energies[basis] =   2*energy(basis, ch4) \
- 2*energy(basis, h2) - energy(basis, c2h4)
if (ga_nodeid() == 0): print basis, ' %8.6f' % energies[basis]
end



In this example the reaction energy for $2H_2 + C_2H_4 \rightarrow 2CH_4$ is evaluated using MP2 in several basis sets. The geometries are fixed, but could be re-optimized in each basis. To illustrate the useful associative arrays in Python, the reaction energies are put into the associative array energies -- note its declaration at the top of the program.

### Using the database

python
rtdb_put("test_int2", 22)
rtdb_put("test_int", [22, 10, 3],    INT)
rtdb_put("test_dbl", [22.9, 12.4, 23.908],  DBL)
rtdb_put("test_str", "hello", CHAR)
rtdb_put("test_logic", [0,1,0,1,0,1], LOGICAL)
rtdb_put("test_logic2", 0, LOGICAL)

rtdb_print(1)

print "test_str    = ", rtdb_get("test_str")
print "test_int    = ", rtdb_get("test_int")
print "test_in2    = ", rtdb_get("test_int2")
print "test_dbl    = ", rtdb_get("test_dbl")
print "test_logic  = ", rtdb_get("test_logic")
print "test_logic2 = ", rtdb_get("test_logic2")
end



This example illustrates how to access the database from Python.

### Handling exceptions from NWChem

geometry; he 0 0 0; he 0 0 2; end
basis; he library 3-21g; end
scf; maxiter 1; end

python
try:
except NWChemError, message:
print 'Error from NWChem ... ', message
end



The above test program shows how to handle exceptions generated by NWChem by forcing an SCF calculation on He2 to fail due to insufficient iterations.

If an NWChem command fails it will raise the exception "NWChemError" (case sensitive) unless the error was fatal. If the exception is not caught, then it will cause the entire Python program to terminate with an error. This Python program catches the exception, prints out the message, and then continues as if all was well since the exception has been handled.

If your Python program detects an error, raise an unhandled exception. Do not call exit(1) since this may circumvent necessary clean-up of the NWChem execution environment.

### Accessing geometry information -- a temporary hack

In an ideal world the geometry and basis set objects would have full Python wrappers, but until then a back-door solution will have to suffice. We've already seen how to use input_parse() to put geometry (and basis) data into NWChem, so it only remains to get the geometry data back after it has been updated by a geometry optimzation or some other operation.

The following Python procedure retrieves the coordinates in the same units as initially input for a geometry of a given name. Its full source is included in the NWChem contrib directory.

def geom_get_coords(name):
try:
actualname = rtdb_get(name)
except NWChemError:
actualname = name
coords = rtdb_get('geometry:' + actualname + ':coords')
units  = rtdb_get('geometry:' + actualname + ':user units')
if (units == 'a.u.'):
factor = 1.0
elif (units == 'angstroms'):
factor = rtdb_get('geometry:'+actualname+':angstrom_to_au')
else:
raise NWChemError,'unknown units'
i = 0
while (i < len(coords)):
coords[i] = coords[i] / factor
i = i + 1
return coords


A geometry with name NAME has its coordinates (in atomic units) stored in the database entry geometry:NAME:coords. A minor wrinkle here is that indirection is possible (and used by the optimizers) so that we must first check if NAME actually points to another name. In the program this is done in the first try...except sequence. With the actual name of the geometry, we can get the coordinates. Any exceptions are passed up to the caller. The rest of the code is just to convert back into the initial input units -- only atomic units or Angstrøms are handled in this simple example. Returned is a list of the atomic coordinates in the same units as your initial input.

The routine is used as follows

   coords = geom_get_coords('geometry')


or, if you want better error handling

   try:
coords = geom_get_coords('geometry')
except NWChemError,message:
else:
print coords


This is very dirty and definitely not supported from one release to another, but, browsing the output of rtdb_print() at the end of a calculation is a good way to find stuff. To be on safer ground, look in the programmers manual since some of the high-level routines do pass data via the database in a well-defined and supported manner. Be warned -- you must be very careful if you try to modify data in the database. The input parser does many important things that are not immediately apparent (e.g., ensure the geometry is consistent with the point group, mark the SCF as not converged if the SCF options are changed, ...). Where at all possible your Python program should generate standard NWChem input and pass it to input_parse() rather than setting parameters directly in the database.

### Scaning a basis exponent yet again -- plotting and handling child processes

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

print none

python
import Gnuplot, time, signal

def energy_at_exponent(exponent):
input_parse('''
basis noprint
H library 3-21g; O library 3-21g; O d; %f 1.0
end
''' % (exponent))

data = []
exponent = 0.5
while exponent <= 0.6:
energy = energy_at_exponent(exponent)
print ' exponent = ', exponent, ' energy = ', energy
data = data + [[exponent,energy]]
exponent = exponent + 0.02

if (ga_nodeid() == 0):
signal.signal(signal.SIGCHLD, signal.SIG_DFL)
g = Gnuplot.Gnuplot()
g('set style data linespoints')
g.plot(data)
time.sleep(30)  # 30s to look at the plot

end



This illustrates how to handle signals from terminating child processes and how to generate simple plots on UNIX systems. The scanning example is modified so that instead of writing the data to a file for subsequent visualization, it is saved for subsequent visualization with Gnuplot (you'll need both Gnuplot and the corresponding package for Python in your PYTHONPATH. Look at http://monsoon.harvard.edu/~mhagger/download).

The issue is that NWChem traps various signals from the O/S that usually indicate bad news in order to provide better error handling and reliable clean-up of shared, parallel resources. One of these signals is SIGCHLD which is generated whenever a child process terminates. If you want to create child processes within Python, then the NWChem handler for SIGCHLD must be replaced with the default handler. There seems to be no easy way to restore the NWChem handler after the child has completed, but this should have no serious side effect.

## Troubleshooting

Common problems with Python programs inside NWChem.

1. You get the message

0:python_input: indentation must be >= that of first line: 4

This indicates that NWChem thinks that a line is less indented than the first line. If this is not the case then perhaps there is a tab in your input which NWChem treats as a single space character but appears to you as more spaces. Try running untabify in Emacs. The problem could also be the END directive that terminates the PYTHON compound directive -- since Python also has an end statement. To avoid confusion the END directive for NWChem must be at the start of the line.

2. Your program hangs or deadlocks -- most likely you have a piece of code that is restricted to executing on a subset of the processors (perhaps just node 0) but is calling (perhaps indirectly) a function that must execute on all nodes.