# Basis set conventions¶

IOData can load molecular orbital coefficients, density matrices and atomic orbital basis sets from various file formats, and it can also write orbitals and the basis sets in the Molden format. To achieve an unambiguous numerical representation of these objects, conventions for the ordering basis functions (within one shell) and normalization of Gaussian primitives must be fixed.

IOData does not use hard-coded conventions but keeps track of them in attributes of them in IOData.obasis. This attribute is an instance of the iodata.basis.MolecularBasis class, of which the conventions and primitive_normalization attributes contain all the relevant information.

For the time being, the primitive_normalization is always set to 'L2', meaning that the contraction coefficients assume L2-normalized Gaussian primitives. However, IOData does not enforce normalized contractions.

The first subsection provides a mathematical definition of the Gaussian basis functions, which is followed by the specification of the conventions attribute of the MolecularBasis class.

## Gaussian basis functions¶

IOData supports contracted Gaussian basis functions, which have in general the following form:

$b(\mathbf{r}; D_1, \ldots, D_k, P, \alpha_1, \ldots, \alpha_K, \mathbf{r}_A) = \sum_{k=1}^K D_k N(\alpha_k, P) P(\mathbf{r} - \mathbf{r}_A) \exp(-\alpha_k \Vert \mathbf{r} - \mathbf{r}_A \Vert^2)$

where $$K$$ is the contraction length, $$D_k$$ is a contraction coefficient, $$N$$ is a normalization constant, $$P$$ is a Cartesian polynomial, $$\alpha_k$$ is an exponent and $$\mathbf{r}_A$$ is the center of the basis function. The summation over $$k$$ is conventionally called a contraction of primitive Gaussian basis functions. The L2-normalization of each primitive depends on both the polynomial and the exponent and is defined by the following relation:

$\int \Bigl\vert N(\alpha_k, P) P(\mathbf{r} - \mathbf{r}_A) \exp(-\alpha_k \Vert \mathbf{r} - \mathbf{r}_A \Vert^2) \Bigr\vert^2 d\mathbf{r} = 1$

Two types of polynomials will be defined below: Cartesian and pure (harmonic) basis functions.

### Cartesian basis functions¶

When the polynomial consists of a single term as follows:

$P(x,y,z) = x^{n_x} y^{n_y} z^{n_z}$

with $$n_x$$, $$n_y$$, $$n_z$$, zero or positive integer powers, one speaks of Cartesian Gaussian basis functions. One refers to the sum of the powers as the angular momentum of the Cartesian Gaussian basis.

The normalization constant of a primitive function is:

$N(\alpha_k, n_x, n_y, n_z) = \sqrt{\frac {(2\alpha_k/\pi)^{3/2} (4\alpha_k)^{n_x+n_y+n_z}} {(2n_x-1)!! (2n_y-1)!! (2n_z-1)!!} }$

In practice one combines all basis functions of a given angular momentum (or algebraic order) into one shell. A basis specification typically only mentions the total angular momentum, and it is assumed that all polynomials of that order are included in the basis set. The number of basis functions, i.e. the number of polynomials, for a given angular momentum, $$\ell=n_x+n_y+n_z$$, is $$(\ell+1)(\ell+2)/2$$.

### Pure or harmonic basis functions¶

When the polynomial is a real regular solid harmonic, one speaks of pure Gaussian basis functions:

$P(r,\theta,\phi) = C_{\ell m}(r,\theta,\phi) \quad \text{or} \quad P(r,\theta,\phi) = S_{\ell m}(r,\theta,\phi)$

where $$C_{\ell m}$$ and $$S_{\ell m}$$ are cosine- and sine-like real regular solid harmonics, defined for $$\ell \ge 0$$ as follows:

$\begin{split}C_{\ell 0}(r,\theta,\phi) &= R_\ell^0(r,\theta,\phi) \\ C_{\ell m}(r,\theta,\phi) &= \sqrt{2} (-1)^m \operatorname{Re} R_\ell^m(\theta,\phi) \quad m = 1\ldots \ell \\ S_{\ell m}(r,\theta,\phi) &= \sqrt{2} (-1)^m \operatorname{Im} R_\ell^m(\theta,\phi) \quad m = 1\ldots \ell\end{split}$

where $$R_\ell^m$$ are the regular solid harmonics, which have in general complex function values. The factor $$(-1)^m$$ undoes the Condon-Shortley phase. In these equations, spherical coordinates are used:

$\begin{split}x &= R\sin\theta\cos\phi \\ y &= R\sin\theta\sin\phi \\ z &= R\cos\theta\end{split}$

The regular solid harmonics are derived from the standard spherical harmonics, $$Y_\ell^m$$, as follows:

$\begin{split}R_\ell^m(r, \theta, \varphi) &= \sqrt{\frac{4\pi}{2\ell+1}} \, r^\ell \, Y_\ell^m(\theta, \varphi) \\ &= \sqrt{\frac{(\ell-m)!}{(\ell+m)!}} \, r^\ell \, P_\ell^m(\cos{\theta}) \, e^{i m \varphi}\end{split}$

where $$P_\ell^m$$ are the associated Legendre functions. After substituting this definition of the regular solid harmonics into the real forms, one obtains:

$\begin{split}C_{\ell 0}(r,\theta,\phi) & = P_\ell^0(\cos{\theta}) \, r^\ell \\ C_{\ell m}(r,\theta,\phi) & = (-1)^m \sqrt{\frac{2(\ell-m)!}{(\ell+m)!}} \, r^\ell \, P_\ell^m(\cos{\theta}) \, \cos(m \phi) \quad m = 1\ldots \ell \\ S_{\ell m}(r,\theta,\phi) & = (-1)^m \sqrt{\frac{2(\ell-m)!}{(\ell+m)!}} \, r^\ell \, P_\ell^m(\cos{\theta}) \, \sin(m \phi) \quad m = 1\ldots \ell \\\end{split}$

Also here, the factor $$(-1)^m$$ cancels out the Condon-Shortley phase. These expressions show that cosine-like functions contain a factor $$\cos(m \phi)$$, and similarly the sine-like functions contain a factor $$\sin(m \phi)$$. The factor $$r^\ell$$ causes real regular solid harmonics to be homogeneous Cartesian polynomials, i.e. linear combinations of the Cartesian polynomials defined in the previous subsection.

Real regular solid harmonics are used because the pure s- and p-type functions are consistent with their Cartesian counterparts:

$\begin{split}C_{00}(x,y,z) & = 1 \\ C_{10}(x,y,z) & = z \\ C_{11}(x,y,z) & = x \\ S_{11}(x,y,z) & = y \\ \dots &\end{split}$

The normalization constant of a pure Gaussian basis function is:

$N(\alpha_k, \ell) = \sqrt{\frac {(2\alpha_k/\pi)^{3/2} (4\alpha_k)^\ell} {(2\ell-1)!!} }$

In practical applications, all the basis functions of a given angular momentum are used and grouped into a shell. A basis specification typically only mentions the total angular momentum, and it is assumed that all polynomials of that order are included in the basis set. The number of basis functions, i.e. the number of polynomials, for a given angular momentum, $$\ell$$, is $$2\ell+1$$.

## The conventions attribute¶

Different file formats supported by IOData have an incompatible ordering of basis functions within one shell. Also the sign conventions may differ from the definitions given above. The conventions attribute of iodata.basis.MolecularBasis specifies the ordering and sign flips relative to the above definitions. It is a dictionary,

• whose keys are tuples denoting a shell type (angmom, char) where angmom is a positive integer denoting the angular momentum and char is either 'c' or 'p' for Cartesian are pure, respectively

• and whose values are lists of basis function strings, where each string denotes one basis function.

A basis function string has a one-to-one correspondence to the Cartesian or pure polynomials defined above.

• In case of Cartesian functions, $$x^{n_x} y^{n_y} z^{n_z}$$ is represented by the string 'x' * nx + 'y' * ny + 'z' * nz, except for the s-type function, which is represented by '1'.

• In case of pure functions, $$C_{\ell m}$$ is represented by 'c{}'.format(m) and $$S_{\ell m}$$ is by 's{}'.format(m). The angular momentum quantum number is not included because it is implied by the key in the conventions dictionary.

Each basis function string can be prefixed with a minus sign, to denote a sign flip with respect to the definitions on this page. The order of the string in the list defines the order of the corresponding basis functions within one shell.

For example, pure and Cartesian s, p and d functions in Gaussian FCHK files adhere to the following convention:

conventions = {
(0, 'c'): ['1'],
(1, 'c'): ['x', 'y', 'z'],
(2, 'c'): ['xx', 'yy', 'zz', 'xy', 'xz', 'yz'],
(2, 'p'): ['c0', 'c1', 's1', 'c2', 's2'],
}


(Pure s and p functions are never used in a Gaussian FCHK file.)

## Notes on other conventions¶

To avoid confusion, negative magnetic quantum numbers are never used to label pure functions in IOData. The basis strings contain ‘c’ and ‘s’ instead. In the literature, e.g. in the book Molecular Electronic-Structure Theory by Helgaker, Jørgensen and Olsen, negative magnetic quantum numbers for pure functions are usually referring to sine-like functions:

$\begin{split}R_{\ell, m} &= C_{\ell m} \quad m = 0 \ldots \ell \\ R_{\ell, -m} &= S_{\ell m} \quad m = 1 \ldots \ell\end{split}$

Note that $$\ell$$ and $$m$$ both appear as subscripts in $$R_{\ell, m}$$ and $$R_{\ell, -m}$$ to tell them apart from their complex counterparts.

## Transformation from Cartesian to pure functions¶

Pure Gaussian primitives can written as linear combinations of Cartesian ones. Hence, integrals over Cartesian functions can also be transformed into integrals over pure primitives. This transformation is the last step in the calculation of the overlap matrix in IOData:

1. Integrals are first computed for Gaussian primitives without normalization.

2. Normalization constants for Cartesian primitives are multiplied into the integrals.

3. Integrals over primitives are contracted.

4. Optionally, the integrals for Cartesian functions are transformed into integrals for pure functions.

For the last step, pre-computed transformations matrices (generated by tools/harmonics.py are stored in iodata/overlap_cartpure.py using the HORTON2_CONVENTIONS. The derivation of these transformation matrices is explained below.

### Recursive computation of real regular solid harmonics¶

First, we construct two sets of recursion relations for $$\phi$$ and $$\theta$$ separately. These will be combined to form the final set of recursion relations that directly operate on the real regular solid harmonics. In these two sets, the notation $$\rho = \sqrt{x^2 + y^2}$$ is used.

The first set of recursion relations starts from a fairly trivial idea:

$\begin{split}\begin{split} \rho^m [\cos(m\phi) + i\sin(m\phi)] &= \rho^m \exp(im\phi) \\ &= \rho \exp(i\phi) \; \rho^{m-1}\exp(i(m-1)\phi) \\ &= (x + iy) \; \rho^{m-1} [\cos((m-1)\phi) + i\sin((m-1)\phi)] \end{split}\end{split}$
$\begin{split}\rho \cos(\phi) &= x \\ \rho \sin(\phi) &= y \\ \rho \cos(m\phi) &= x \cos((m-1)\phi) - y \sin((m-1)\phi) \\ \rho \sin(m\phi) &= x \sin((m-1)\phi) + y \cos((m-1)\phi)\end{split}$

Second, recursion relations for associated Legendre functions can be modified to contain $$r$$, $$z$$ and $$\rho$$, such that $$\cos\theta$$ does not appear explicitly:

$\begin{split}P_0^0(\cos\theta) &= 1 \\ r^\ell P_\ell^\ell(\cos\theta) &= (2\ell - 1) \rho \; r^{\ell-1} P_{\ell-1}^{\ell-1}(\cos\theta) \\ r^{\ell} P_{\ell}^{\ell-1}(\cos\theta) &= -(2\ell - 1) z \; r^{\ell-1} P_{\ell-1}^{\ell-1}(\cos\theta) \\ r^\ell P_{\ell}^{m}(\cos\theta) &= \frac{2\ell - 1}{\ell - m} z \; r^{\ell-1} P_{\ell-1}^{m}(\cos\theta) -\frac{\ell + m - 1}{\ell - m} r^2 \; r^{\ell-2} P_{\ell-2}^{m}(\cos\theta)\end{split}$

The two sets could be used separately to construct real regular solid harmonics, but they feature $$\rho=\sqrt{x^2+y^2}$$, while the regular solid harmonics should be homogeneous polynomials. We can get rid of $$\rho$$ by combining the two sets into one:

$\begin{split}C_{0,0} ={}& 1 \\ C_{1,0} ={}& z \\ C_{1,1} ={}& x \\ S_{1,1} ={}& y \\ C_{\ell,\ell} ={}& \sqrt{\frac{2\ell-1}{2\ell}} \; \Bigl[x C_{\ell-1,\ell-1} - y S_{\ell-1,\ell-1} \Bigr] \quad \forall \; \ell > 1 \\ S_{\ell,\ell} ={}& \sqrt{\frac{2\ell-1}{2\ell}} \; \Bigl[x S_{\ell-1,\ell-1} + y C_{\ell-1,\ell-1} \Bigr] \quad \forall \; \ell > 1 \\ \{CS\}_{\ell,\ell-1} ={}& z \sqrt{2\ell-1} \; \{CS\}_{\ell-1, \ell-1} \quad \forall \; \ell > 1 \\ \{CS\}_{\ell,m} ={}& \frac{(2\ell - 1)z}{\sqrt{(\ell+m)(\ell-m)}} \{CS\}_{\ell-1,m} \nonumber \\ & - r^2 \sqrt{\frac{(\ell - m - 1)(\ell + m - 1)}{(\ell + m)(\ell - m)}} \{CS\}_{\ell - 2,m} \nonumber \\ & \quad \forall \; \ell > m + 1 \text{ and } m \ge 0\end{split}$

These equations show that real regular solid harmonics are homogeneous polynomials in $$x$$, $$y$$ and $$z$$. Advantages of this approach are (i) the absence of trigonometric expressions and (ii) the similarity between cosine and sine expressions. (Coefficients can be reused.) These recursion relations should be numerically stable for the computation of real regular solid harmonics as a function of Cartesian coordinates. They can also be used to build a transformation matrix from Cartesian mononomials into real regular solid harmonics.

### Transformation matrices without normalization¶

The above recursion relations result in the following transformation matrices. These were obtained by running:

python tools/harmonics.py none latex 3

$\begin{split}\left(\begin{array}{c} b(C_{20}) \\ b(C_{21}) \\ b(S_{21}) \\ b(C_{22}) \\ b(S_{22}) \end{array}\right) &= \left(\begin{array}{cccccc} - \frac{1}{2} & \cdot & \cdot & - \frac{1}{2} & \cdot & 1 \\ \cdot & \cdot & \sqrt{3} & \cdot & \cdot & \cdot \\ \cdot & \cdot & \cdot & \cdot & \sqrt{3} & \cdot \\ \frac{\sqrt{3}}{2} & \cdot & \cdot & - \frac{\sqrt{3}}{2} & \cdot & \cdot \\ \cdot & \sqrt{3} & \cdot & \cdot & \cdot & \cdot \\ \end{array}\right) \left(\begin{array}{c} b(xx) \\ b(xy) \\ b(xz) \\ b(yy) \\ b(yz) \\ b(zz) \end{array}\right) \\ \left(\begin{array}{c} b(C_{30}) \\ b(C_{31}) \\ b(S_{31}) \\ b(C_{32}) \\ b(S_{32}) \\ b(C_{33}) \\ b(S_{33}) \end{array}\right) &= \left(\begin{array}{cccccccccc} \cdot & \cdot & - \frac{3}{2} & \cdot & \cdot & \cdot & \cdot & - \frac{3}{2} & \cdot & 1 \\ - \frac{\sqrt{6}}{4} & \cdot & \cdot & - \frac{\sqrt{6}}{4} & \cdot & \sqrt{6} & \cdot & \cdot & \cdot & \cdot \\ \cdot & - \frac{\sqrt{6}}{4} & \cdot & \cdot & \cdot & \cdot & - \frac{\sqrt{6}}{4} & \cdot & \sqrt{6} & \cdot \\ \cdot & \cdot & \frac{\sqrt{15}}{2} & \cdot & \cdot & \cdot & \cdot & - \frac{\sqrt{15}}{2} & \cdot & \cdot \\ \cdot & \cdot & \cdot & \cdot & \sqrt{15} & \cdot & \cdot & \cdot & \cdot & \cdot \\ \frac{\sqrt{10}}{4} & \cdot & \cdot & - \frac{3 \sqrt{10}}{4} & \cdot & \cdot & \cdot & \cdot & \cdot & \cdot \\ \cdot & \frac{3 \sqrt{10}}{4} & \cdot & \cdot & \cdot & \cdot & - \frac{\sqrt{10}}{4} & \cdot & \cdot & \cdot \\ \end{array}\right) \left(\begin{array}{c} b(xxx) \\ b(xxy) \\ b(xxz) \\ b(xyy) \\ b(xyz) \\ b(xzz) \\ b(yyy) \\ b(yyz) \\ b(yzz) \\ b(zzz) \end{array}\right)\end{split}$

### Taking into account normalization¶

For the calculation of the overlap matrix, the transformations need to be modified, to transform normalized Cartesian functions into normalized pure functions. Accounting for normalization yields slightly different matrices shown below. These were obtained by running:

python tools/harmonics.py L2 latex 3

$\begin{split}\left(\begin{array}{c} b(C_{20}) \\ b(C_{21}) \\ b(S_{21}) \\ b(C_{22}) \\ b(S_{22}) \end{array}\right) &= \left(\begin{array}{cccccc} - \frac{1}{2} & \cdot & \cdot & - \frac{1}{2} & \cdot & 1 \\ \cdot & \cdot & 1 & \cdot & \cdot & \cdot \\ \cdot & \cdot & \cdot & \cdot & 1 & \cdot \\ \frac{\sqrt{3}}{2} & \cdot & \cdot & - \frac{\sqrt{3}}{2} & \cdot & \cdot \\ \cdot & 1 & \cdot & \cdot & \cdot & \cdot \\ \end{array}\right) \left(\begin{array}{c} b(xx) \\ b(xy) \\ b(xz) \\ b(yy) \\ b(yz) \\ b(zz) \end{array}\right) \\ \left(\begin{array}{c} b(C_{30}) \\ b(C_{31}) \\ b(S_{31}) \\ b(C_{32}) \\ b(S_{32}) \\ b(C_{33}) \\ b(S_{33}) \end{array}\right) &= \left(\begin{array}{cccccccccc} \cdot & \cdot & - \frac{3 \sqrt{5}}{10} & \cdot & \cdot & \cdot & \cdot & - \frac{3 \sqrt{5}}{10} & \cdot & 1 \\ - \frac{\sqrt{6}}{4} & \cdot & \cdot & - \frac{\sqrt{30}}{20} & \cdot & \frac{\sqrt{30}}{5} & \cdot & \cdot & \cdot & \cdot \\ \cdot & - \frac{\sqrt{30}}{20} & \cdot & \cdot & \cdot & \cdot & - \frac{\sqrt{6}}{4} & \cdot & \frac{\sqrt{30}}{5} & \cdot \\ \cdot & \cdot & \frac{\sqrt{3}}{2} & \cdot & \cdot & \cdot & \cdot & - \frac{\sqrt{3}}{2} & \cdot & \cdot \\ \cdot & \cdot & \cdot & \cdot & 1 & \cdot & \cdot & \cdot & \cdot & \cdot \\ \frac{\sqrt{10}}{4} & \cdot & \cdot & - \frac{3 \sqrt{2}}{4} & \cdot & \cdot & \cdot & \cdot & \cdot & \cdot \\ \cdot & \frac{3 \sqrt{2}}{4} & \cdot & \cdot & \cdot & \cdot & - \frac{\sqrt{10}}{4} & \cdot & \cdot & \cdot \\ \end{array}\right) \left(\begin{array}{c} b(xxx) \\ b(xxy) \\ b(xxz) \\ b(xyy) \\ b(xyz) \\ b(xzz) \\ b(yyy) \\ b(yyz) \\ b(yzz) \\ b(zzz) \end{array}\right)\end{split}$