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In this tutorial, we will run a CIPSI calculation on the HCN molecule in two basis sets. Before using the quantum package, you need to load the needed environment variables by sourcing the $QP_ROOT/quantum_package.rc file. For example::

source $HOME/quantum_package/quantum_package.rc

Ensure all required binaries are compiled

This command will compile everything and install the Hartree-Fock and Full-CI module

  cd $QP_ROOT
  qp_module.py install Full_CI_ZMQ
  ninja

Create the EZFIO input file

The EZFIO is a hierarchical data format which uses the hierarchy of the file system to organize the data. The EZFIO file is a database which will contain the input and output data. To access the data in the EZFIO file, you can use the provided API, or tools such as qp_edit, or even look at the files present in the directories.

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

Note that you can also input the coordinates using a Z-matrix:

 c
 n   1 nc2
 h   1 hc3         2 hcn3

nc2         1.156
hc3         1.064
hcn3        180.0

Now, this xyz file is used to generate an EZFIO input directory using the qp_create_ezfio_from_xyz command. The main options of that command are

  -b string      Name of basis set.
  [-au]          Input geometry is in atomic units.
  [-c int]       Total charge of the molecule. Default is 0.
  [-cart]        Compute AOs in the Cartesian basis set (6d, 10f, ...)
  [-d float]     Add dummy atoms. x * (covalent radii of the atoms)
  [-m int]       Spin multiplicity (2S+1) of the molecule. Default is 1.
  [-o file]      Name of the created EZFIO file.
  [-p string]    Name of the pseudopotential

To generate hcn.ezfio in the 6-31G basis set, we use::

qp_create_ezfio_from_xyz -b "6-31G" hcn.xyz

With the qp_edit command, you can edit interactively the EZFIO file with all the options available for all the binaries installed.

Run the SCF and Full-CI calculations

First, create the canonical MOs by running an SCF calculation::

  qp_run SCF hcn.ezfio

The expected SCF energy is -92.8278567 au.

In the selected FCI calculation, by default the program will stop when the 1.000.000 most important determinants are selected, or when the PT2 correction is below 1.e-4. To make a quick test, we want to stop for 10.000 determinants. To change this behavior, edit hcn.ezfio to modify the options of the selected Full-CI calculation::

  qp_edit hcn.ezfio

This will open a temporary file enabling the modification of the EZFIO directory. Search for the Determinants section, and set the n_det_max option to 10000::

  n_det_max = 10000

Now we can run the Full-CI calculation. Type

  qp_run fci_zmq hcn.ezfio

The variational energy should be ‘-93.044945’ and the Full-CI approximate energy should be ‘-93.052019’.

Freeze core electrons in the CI

We will enlarge the basis set to cc-pvdz and freeze the core orbitals in the CI calculation.

First create the new EZFIO with the relevant basis-set qp_create_ezfio_from_xyz -b "cc-pvdz" hcn.xyz -o hcn_large.ezfio

Then run the SCF

  qp_run SCF hcn_large.ezfio

The expected SCF energy is -92.8829467.

We want to run the selected Full-CI calculation in the valence only. For this, we will use the qp_set_mo_class utility. The options are

-act range     Range of active orbitals
-core range    Range of core orbitals
-del range     Range of deleted orbitals
-inact range   Range of inactive orbitals
-virt range    Range of virtual orbitals

We set 2 first canonical orbitals as core and all the remaining MOs are set as active ::

  qp_set_mo_class hcn_large.ezfio -core "[1,2]" -act "[3-33]"

or we can do it automatically:

  qp_set_frozen_core.py hcn_large.ezfio

The qp_set_frozen_core.py script finds the heavy atoms and proposes a set of core and active MOs. For atoms with few electrons in the valence such as Li or Na, the script will propose a small core.

Then, edit hcn_large.ezfio to modify the options of the selected Full-CI calculation::

  qp_edit hcn_large.ezfio

This will open a temporary file enabling the modification of the EZFIO directory. Search for the Determinants section, and set the n_det_max option to 10000::

  n_det_max = 10000

And run the Full-CI calculation::

  qp_run fci_zmq hcn_large.ezfio

The expected variational energy is -93.16482111.

From the current wave function, we can generate the corresponding natural orbitals::

  qp_run save_natorb hcn_large.ezfio

and run the selected FCI calculation again. The expected variational energy is -93.172895441 and the energy + the PT2 contribution is -93.19059084.