Quick-start guide¶

This tutorial should teach you everything you need to get started with the the basics of the Quantum Package. As an example, we will run a frozen core CIPSI calculation on the HCN molecule in the 631-G basis set.

Demo video¶

Hands on¶

Important

Before using the Quantum Package, it is required to source the file quantum_package.rc if it has not been done already in the current shell.

Create the EZFIO database¶

The data relative to calculations are stored in an EZFIO database. EZFIO is a hierarchical data format which uses the hierarchy of the file system to organize the data, as files stored in a directory. The data in the EZFIO directory are stored as plain text files, so it can be opened with any text editor. To access the data of the EZFIO database, the APIs (Fortran, Python, OCaml or Bash) provided by EZFIO should be used, or tools using these APIs such as qp_edit provided with the Quantum Package.

First, create an xyz file containing the coordinates of the molecule. The file hcn.xyz contains:

3
HCN molecule
C    0.0    0.0    0.0
H    0.0    0.0    1.064
N    0.0    0.0    -1.156

This xyz file is now used with the qp_create_ezfio command to create an EZFIO database with the 6-31G basis set:

qp_create_ezfio -b "6-31G" hcn.xyz -o hcn

The EZFIO database now contains data relative to the nuclear coordinates and the atomic basis set:

$ ls hcn
ao_basis/  electrons/  ezfio/  nuclei/  pseudo/

Run a Hartree-Fock calculation¶

The program qp_run is the driver program of the Quantum Package. To run a SCF calculation, just run

qp_run scf hcn

The expected energy is -92.827856698 au.

Choose the target MO space¶

Now, modify to EZFIO database to make CIPSI calculation in the full set of valence MOs, keeping the core MOs frozen. The simple command qp_set_frozen_core does this automatically:

qp_set_frozen_core hcn

The general command to specify core and active orbitals is qp_set_mo_class. In the case of HCN molecule in the 631G basis, one has 20 MOs in total and the two first orbitals are frozen:

qp_set_mo_class -core "[1-2]" -act "[3-20]" hcn

Run the CIPSI calculation¶

We will now use the CIPSI algorithm to estimate the FCI energy.

qp_run fci hcn | tee hcn.fci.out

The program will start with a single determinant and will iteratively:

Select the most important determinants from the external space and add them to the internal space
Add all the necessary determinants to allow the eigenvector of \(\hat H\) to be also an eigenstate of \(\widehat{S^2}\)
Diagonalize \(\hat H\) in the enlarged internal space
Compute (stochastically) the second-order perturbative contribution to the energy
Extrapolate the variational energy by fitting \(E=E_\text{FCI} - \alpha\, E_\text{PT2}\)

By default, the program will stop when more than one million determinants have entered in the internal space, or when the PT2 energy is below \(10^{-4}\).

To have a pictural illustration of the convergence of the CIPSI algorithm, just run

qp_e_conv_fci hcn.fci.out

This will create the files “hcn.fci.out.conv” containing the data of the convergence of the energy that can be plotted, together with the file “hcn.fci.out.conv.1.eps” which is obtained from the gnuplot plot file “hcn.fci.out.conv.plt”.

The estimated FCI energy of HCN is -93.0501 au.