qmcpack/examples/molecules/He

Input files for a single helium atom.


Simplest correlated wavefunction
--------------------------------
he_simple.xml 

The simplest possible wavefunction with electron correlation.  It uses
Slater Type Orbitals (STO) combined with a Pade form for the electron-electron
Jastrow.  The exponent of the STO should be fixed to the nuclear charge (Z=2)
to cancel the -Z/r Coulomb term between the electrons and the nucleus (cusp
condition).  The Jastrow factor has one adjustable parameter (b).
This runs Variational Monte Carlo (VMC) to compute the energy.

Analyzing QMCPACK output
------------------------
One the output files, `He.s000.scalar.dat` contains scalar values, such as
the local energy (first column).  There should be one line of data for each
block (the file should have `<blocks>` lines, plus one for the header).

To analyze this data, use the `qmca` script (in the `nexus/executables` directory)
on the scalar data file: `qmca He.s000.scalar.dat`.  It should print out average
values for each scalar along with an error estimate.

The `qmca` script has other features (use `qmca -h` to see the help).
For instance, to plot a trace of the local energy (matplotlib must be installed):
`qmca --quantities LocalEnergy --trace He.s000.scalar.dat`.

 

Optimizing variational parameters
---------------------------------
he_simple_opt.xml

This file performs 10 iterations of the optimization loop.  After running with
QMCPACK, there should be a number of files in the directory with different
 series (the part the filename with `s000`, `s001`, etc.).   Series 000-009
are the optimization iterations, and series 010 is the final VMC run.
The file `He.s009.opt.xml` contains the wavefunction with optimized parameters.

The `qmca` script can plot the average energy for each run.
Use `qmca --plot --quantities LocalEnergy He.s0*.scalar.dat`.  Because this
is a simple wavefunction with one parameter, only the first iteration really
reduced the energy.


Diffusion Monte Carlo
---------------------
he_simple_dmc.xml

Runs Diffusion Monte Carlo (DMC).  It first runs VMC to generate a set of
walkers, and then performs DMC.  There will be two series of output files - 000
for the VMC and 001 for the DMC.

Since there is only one electron of each spin, DMC should reproduce the exact
non-relativistic ground state energy (-2.90372 Hartree).
 

B-Splines
---------
he_bspline_jastrow.xml

This uses a B-spline form for the electron-electron Jastrow correlation. There
are four knots in the B-spline, and so four variational parameters.  The input
is set to optimize the B-spline coefficients.


Orbitals from GAMESS
--------------------
he.inp
he_from_gamess.xml

The orbitals were obtained from the output of GAMESS.  The GAMESS input file
is he.inp.

The `he_from_gamess.xml` input file does a VMC run by default.  The optimization
input code is commented out.  Uncomment it and run to optimize the parameters.
The number of samples (`<parameter name="samples">`) may need to be increased
for better optimization convergence.