diff --git a/trex.org b/trex.org
index c9824ea..7b93fdf 100644
--- a/trex.org
+++ b/trex.org
@@ -684,13 +684,19 @@ power = [
    \]
 
    where $i$ is the atomic orbital index, $P$ refers to either
-   polynomials or spherical harmonics, and $s(i)$ specifies the shell
-   on which the AO is expanded.
+   polynomials in $x,y,z$ or real solid harmonics
+   \[
+   S^m_{\ell}(\mathbf{r}) \equiv \sqrt{\frac{4\pi}{2\ell+1}}\; r^\ell
+   Y^m_{\ell}(\theta,\varphi)
+   \]
+   (see [[https://en.wikipedia.org/wiki/Solid_harmonics][Wikipedia]]), and $s(i)$
+   specifies the shell on which the AO is expanded.
 
+   
    $\eta(i)$ denotes the chosen angular function.  The AOs can be
-   expressed using real spherical harmonics or polynomials in Cartesian
-   coordinates.  In the case of real spherical harmonics, the AOs are
-   ordered as $0, +1, -1, +2, -2, \dots, + m, -m$ (see [[https://en.wikipedia.org/wiki/Table_of_spherical_harmonics#Real_spherical_harmonics][Wikipedia]]).  In
+   expressed using real solid harmonics or polynomials in Cartesian
+   coordinates.  In the case of real solid harmonics, the AOs are
+   ordered as $0, +1, -1, +2, -2, \dots, + m, -m$).  In
    the case of polynomials, the canonical (or alphabetical) ordering is
    used,