Added plane waves

ChangeLog

@@ -12,6 +12,7 @@ CHANGES
 - Added `float buffered` type for vectors like CI/CSF coefficients
 - .git folder is no longer needed to activate TREXIO_DEVEL mode
 - Renamed debian folder into helpers-debian
+- Added plane wave basis set
 - Added trexio_to_bitfield_list functionality
 - Added `trexio_has_group` functionality
 - Added OCaml binding

trex.org

@@ -276,6 +276,9 @@ power = [
 
 * Basis set (basis group)
 
+
+** Gaussian and Slater-type orbitals
+
    We consider here basis functions centered on nuclei. Hence, we enable
    the possibility to define /dummy atoms/ to place basis functions in
    random positions.
@@ -310,13 +313,26 @@ power = [
    implies the computation of an extra normalization factor, $\mathcal{N}_s$.
    If the the basis function is not considered normalized, $\mathcal{N}_s=1$.
 
+   All the basis set parameters are stored in one-dimensional arrays.
 
-  All the basis set parameters are stored in one-dimensional arrays:
+** Plane waves
+
+   A plane wave is defined as
+
+   \[
+      \chi_j(r) = \exp \left( -i \mathbf{k}_j \mathbf{r} \right)
+   \]
+
+   The basis set is defined as the array of $k$-points in the
+   reciprocal space, defined in the ~pbc~ group. The kinetic energy
+   cutoff ~e_cut~ is the only input data relevant to plane waves.
+
+** Data definitions
    
    #+NAME: basis
    | Variable        | Type    | Dimensions          | Description                                                     |
-  |-----------------+---------+---------------------+--------------------------------------------------------------|
+   |-----------------+---------+---------------------+-----------------------------------------------------------------|
-  | ~type~          | ~str~   |                     | Type of basis set: "Gaussian" or "Slater"                    |
+   | ~type~          | ~str~   |                     | Type of basis set: "Gaussian", "Slater" or "PW" for plane waves |
    | ~prim_num~      | ~dim~   |                     | Total number of primitives                                      |
    | ~shell_num~     | ~dim~   |                     | Total number of shells                                          |
    | ~nucleus_index~ | ~index~ | ~(basis.shell_num)~ | One-to-one correspondence between shells and atomic indices     |
@@ -327,6 +343,8 @@ power = [
    | ~exponent~      | ~float~ | ~(basis.prim_num)~  | Exponents of the primitives ($\gamma_{ks}$)                     |
    | ~coefficient~   | ~float~ | ~(basis.prim_num)~  | Coefficients of the primitives ($a_{ks}$)                       |
    | ~prim_factor~   | ~float~ | ~(basis.prim_num)~  | Normalization coefficients for the primitives ($f_{ks}$)        |
+   | ~e_cut~         | ~float~ |                     | Energy cut-off for plane-wave calculations                      |
+
 
    #+CALL: json(data=basis, title="basis")
 
@@ -345,6 +363,7 @@ power = [
          ,      "exponent" : [ "float", [ "basis.prim_num" ]  ]
          ,   "coefficient" : [ "float", [ "basis.prim_num" ]  ]
          ,   "prim_factor" : [ "float", [ "basis.prim_num" ]  ]
+         ,         "e_cut" : [ "float", []                    ]
        } ,
    #+end_src
    :end:
@@ -917,12 +936,15 @@ prim_factor =
 
 * Cell (cell group)
 
+  3 Lattice vectors to define a box containing the system, for example
+  used in periodic calculations.
+  
   #+NAME: cell
   | Variable | Type    | Dimensions | Description           |
-  |----------+---------+------------+-------------------------|
+  |----------+---------+------------+-----------------------|
-  | ~a~      | ~float~ | ~(3)~      | First unit cell vector  |
+  | ~a~      | ~float~ | ~(3)~      | First lattice vector  |
-  | ~b~      | ~float~ | ~(3)~      | Second unit cell vector |
+  | ~b~      | ~float~ | ~(3)~      | Second lattice vector |
-  | ~c~      | ~float~ | ~(3)~      | Third unit cell vector  |
+  | ~c~      | ~float~ | ~(3)~      | Third lattice vector  |
 
   #+CALL: json(data=cell, title="cell")
 
@@ -939,11 +961,14 @@ prim_factor =
 
 * Periodic boundary calculations (pbc group)
 
+  A single $k$-point per TREXIO file can be stored. The $k$-point is
+  defined in this group.
+  
   #+NAME: pbc
   | Variable      | Type    | Dimensions | Description             |
-  |------------+---------+------------+-------------------------|
+  |---------------+---------+------------+-------------------------|
-  | ~periodic~ | ~int~   |            | ~1~: true or ~0~: false |
+  | ~is_periodic~ | ~int~   |            | ~1~: true or ~0~: false |
-  | ~k_point~  | ~float~ | ~(3)~      | k-point sampling        |
+  | ~k_point~     | ~float~ | ~(3)~      | $k$-point sampling      |
 
   #+CALL: json(data=pbc, title="pbc")