Basic Usage and Examples of QMC=Chem

This section provides a comprehensive guide on how to perform basic operations and run simple examples using QMC=Chem.

Example of a QMC=Chem Calculation

Create the EZFIO Database

Begin by creating an xyz file with the nuclear coordinates of the system. For a water molecule:
```
$ cat > h2o.xyz << EOF
3
Water molecule
O 0. 0. 0.
H 0.9572 0. 0.
H -0.239987 0.926627 0.
EOF
```
Select Basis Set and Pseudopotential

Choose an appropriate basis set and pseudopotential, then create the EZFIO database:
```
$ qp_create_ezfio -b cc-pvdz_ecp_bfd -p bfd h2o.xyz -o h2o
```
Run SCF and CIPSI Calculations

Perform the SCF calculation, followed by a CIPSI calculation:
```
$ qp_run scf h2o
$ qp_run fci h2o
```
Transform Input for QMC=Chem

Prepare the input for QMC=Chem:
```
$ qp_run save_for_qmcchem h2o
```

Load Environment Variables

Before starting, load the QMC=Chem environment:
```
$ source qmcchemrc
```
Understanding the ``qmcchem`` Command

The qmcchem command is central to operations in QMC=Chem. Running it without arguments displays a help message:
```
$ qmcchem
[Output showing various subcommands]
```
Set Parameters for VMC Calculation

Configure the VMC calculation to create initial walker positions:
```
$ qmcchem edit h2o -f 1 -m VMC -n 1.e-5 -s Langevin -t 300 -l 10
```
Run the VMC Calculation

Execute the VMC calculation:
```
$ qmcchem run h2o
```

Configure FN-DMC Parameters

Adjust the parameters for the FN-DMC calculation:

$ qmcchem edit h2o -e -76.438 -m DMC -s Brownian -ts 3.e-4 -t 3600 -l 30

Execute FN-DMC Calculation

Perform the FN-DMC calculation:
```
$ qmcchem run h2o
```
View Results

Finally, view the results of the calculation:
```
$ qmcchem result h2o
```