Tip

All input files can be downloaded: Files.

basinfo

This keyword defines the implementation details of basis functions.

Options

angular_type

Value	Cartesian. Will use Cartesian basis sets, i.e., 6d/10f/15g.
	spherical. Will use the spherical basis sets, i.e., 5d/7f/9g.
Default	spherical

Determine whether to use spherical or Cartesian GTO basis set. Usually, there is NO need to use Cartesian GTO basis sets except for test purpose.

linear_dependence

Value	A real number
Default	1.E-8

Assign the threshold that admits the linear dependence of the basis set. It should be a positive real number. When linear dependence is detected, Qbics will stop the calculation and raises an error.

Linear dependence occurs when:

1. There are identical basis functions for the same nuclear;

2. Two nuclei of the same element are too close;

3. There are many diffuse basis functions for a large molecule.

ghost

Value	Atom range
Default	None

Assigning “ghost atoms” means that those atoms defined in mol ... end have basis functions, but nuclear and electrons disappear. This is often used for some special purposes, like basis set superposition error (BSSE) calculations, although Qbics can automatically do this.

Theoretical Background

Angular Type of Basis Functions

The analyitcal expression of Gaussian basis function is:

\[\chi(\mathbf{r}) = A_{L}(\mathbf{r})\sum_{k=1}^{K} C_k e^{-\alpha_k r_{A}^2}\]

Here, \(A_{L}(\mathbf{r})\) is the angular part with angular momentum quantum number \(L\), \(K\) is the contraction degree, \(\alpha_k\) is the exponent, \(C_k\) is the contraction coefficient, and \(\mathbf{A}\) is the atom position.

The angular part \(A_{L}(\mathbf{r})\) has 2 forms: one is Cartesian, of the form \(x_A^{l_x}y_A^{l_y}z_A^{l_z}\), and spherical, of a real solid spherical harmonic function. The spherical basis sets are more efficient and compact for quantum chemical calculations.

Ghost Atoms

In quantum chemistry, ghost atoms are atoms that have basis functions but no nuclear and electrons. They are often used in basis set superposition error (BSSE) calculations, where the interaction between two molecules is calculated. The ghost atoms are used to remove the interaction between the basis functions of the two molecules.

One can use ghost option to calculate BSSE manually, but this is unnecessary. Qbics can automatically handle this by eda.

Input Examples

Example: GeH₃F-NCH with Ghost Atoms

For the complex GeH₃F-NCH, calculate the energy that HCN is the ghost molecule at B3LYP/def2-SVP level of theory:

basinfo-1.inp

basis
    def2-svp
end

basinfo
    ghost 6-8 # The indices of HCN.
end

scf
    charge  0
    spin2p1 1
    type    R
end

mol
   Ge      0.00000000      0.00221863     -0.79935317
    H      0.00000000      1.48645043     -0.40384625
    H      1.28514604     -0.74161126     -0.40477816
    H     -1.28514603     -0.74161126     -0.40477816
    F      0.00000000      0.00108752     -2.56116087
    C      0.00000000     -0.00225138      3.35662076
    H      0.00000000     -0.00220444      4.43604901
    N      0.00000000     -0.00207825      2.20326200
end

task
    energy b3lyp
end

The ghost keyword indicates that atom 6,7,8, i.e., C,H,N are ghost ones.

In the output, you can see:

basinfo-1.out

# of ghost atoms: 3
 Indices: 6 7 8
 Sum of ghost atom electrons: 14

...

SCF Structure:
 # of electrons:       44
 # of alpha electrons: 22
 # of beta electrons:  22
 2S+1:                 1

The ghost atoms are listed. Also, the number of electrons are only 44, which is exactly the number of electrons for GeH₃F only.