basis
=====

This option defines the basis functions used for quantum chemistry calculations. You can define basis sets in several flexible ways.

.. contents::
   :local:

Using Built-in Basis Sets
-------------------------

A lot of important basis sets have been provided in a directory ``basis`` in the same path of Qbics. The files are named after their names well-known in computational chemistry community. For example, ``basis/cc-pvdz`` contains the cc-pVDZ basis. All files are named in small cases.

To use them, simple write down the basis set name. It is case-insensitive. For example, to use def2-TZVP for all atoms:

.. code-block:: bash
    :linenos:

    basis
      def2-TZVP
    end

Qbics will extract basis set information from ``basis/def2-tzvp``.

Explicit Basis Set Definitions
------------------------------

You can also explicitly define your own basis sets. For example, your system contains two atoms, H and Li, then their basis sets can be defined in this way:

.. code-block:: bash
    :linenos:

    basis 
      H     0
      S   3   1.00
            13.0107010             0.19682158E-01
            1.9622572              0.13796524
            0.4445379              0.47831935
      S   1   1.00
            0.12194962             1.0000000
      P   1   1.00
            0.8000000              1.0000000
      ****
      Li     0
      S   5   1.00
            266.27785516           0.64920150325E-02
            40.069783447           0.47747863215E-01
            9.0559944389           0.20268796111
            2.4503009051           0.48606574817
            0.72209571855          0.43626977955
      S   1   1.00
            0.52810884721E-01      1.0000000
      S   1   1.00
            0.20960948798E-01      1.0000000
      P   2   1.00
            1.4500000              0.2586000
            0.3000000              1.0000000
      P   1   1.00
            0.0820000              1.0000000
      ****
    end

The basis set definition is of standard Gaussian94 format:

- The definition of the basis set for each atom ends with 4 asterisks, i.e.  ``****``.
- The definition starts with the element name like ``Li`` and a ``0``. Currently ``0`` has no meaning.
- Then, each GTO shell is defined. The shell definition starts with three parameters:  

  * Angular momentum. It can be any nonnegative numbers like ``0``, ``5``, or one of ``S``, ``P``, ``D``, ``F``, ``G``, ``H``, and ``I``.
  * Contraction degree. It must be a positive integer.
  * A real number. Currently it has no meaning.
  * Then, each line defines the exponent and contraction coefficient of the primitive GTO to be contracted. They are 2 real numbers.

.. hint:: 
    Note that many basis sets in Gaussian94 format can be obtained from https://www.basissetexchange.org/. But, remember to replace ``D`` to ``E`` since the former is not recognized by Qbics.

Using Self-defined Basis Set Files
----------------------------------

You can also put your explicit basis set definitions into some files, say ``/home/zhang/userdef/my-own-basis``. Qbics will automatically read it if you give explicit file name including path.

.. code-block:: bash
     :linenos:

     basis
       /home/zhang/userdef/my-own-basis
     end 

Define Different Basis Sets for Different Elements
---------------------------------------------------

If you want to use different basis set s for different elements, then you can write ``element`` in the first line, then write element and basis set file name line by line. For example:

.. code-block:: bash
     :linenos:

      basis
        element # This indicates that Qbics will assign basis set element by element.
        O aug-cc-pvtz
        C cc-pvdz
        N /home/zhang/userdef/my-own-basis
      end