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.