Diffusion Monte Carlo

Let us start to run a DMC simulation with the HF orbitals and the optimal Jastrow factor you have just generated.

Create a new directory and copy the wave function TREXIO info and the optimal Jastrow factor (for simplicity, pick the last one).

First, generate an input file as before where you read the wave function files (careful to load the new Jastrow factor) and perform a short VMC calculation to generate the walkers for DMC.

To shorten the VMC run, you can choose a small vmc_nblk in the main input file and modify vmc_nconf_new to be the number of walkers per core you wish. Here, we use the same values as for the starting iterations of the Jastrow factor optimization:

%module blocking_vmc
    vmc_nstep     20
    vmc_nblk      200
    vmc_nblkeq    1
    vmc_nconf_new 100
%endmodule

This will generate 100 walkers per core (vmc_nconf_new) by writing the coordinates of a walker every \(20 \times 200 / 100\) steps. Since the correlation time is less than 2 step in VMC, your walkers will be decorrelated.

A bunch of mc_configs_newX files will appear in your directory, each containing 100 walkers.

cat mc_configs_new* >> mc_configs
rm mc_configs_new*

mc_configs contains now all walkers.

Generate a DMC input

%module blocking_dmc
    dmc_nstep     60
    dmc_nblk      40
    dmc_nblkeq    1
    dmc_nconf     100
%endmodule

%module dmc
    tau           0.05
    etrial      -17.240
    icasula      -1
%endmodule

You also need to change the mode keyword in the input file:

mode            'dmc_one_mpi1'

within the general module.

Some debug files are being created, that you can just erase.

rm problem*
rm mc_configs_new*

To look at the energy, you can do

grep '( 100) =' dmc*out

In the last column, you have the correlation time.

Make sure that you have chosen dmc_nstep about two times larger.

Also perform another calculation with a smaller time step.

Make sure that you increase dmc_nstep by as much as you have decreased \(\tau\).

You do not need to regenerate the file mc_configs containing the walkers.

Repeat the optimization and DMC calculation with the DFT orbitals. Compare the VMC and DMC energies.

Optimal one-determinant Jastrow-Slater wave function

Finally, starting from the DFT orbitals and the optimal two-body Jastrow optimize the full wave function (Jastrow and orbitals).

To this aim, set ioptorb to 1 in the optwf module.

ioptorb     1

More examples to play with

Multiple files with geometries, basis sets and pseudopotentials can be downloaded here: Examples