Keywords for Periodic Electrostatic Embedded Cluster Method

The basic PEECM settings are defined in the `$embed` block.
It can be redirected to an external file using `$embed file=`

SPMlt;file_name>;.

Following keywords are used for the PEECM calculation setup:

periodicSpecifies the number of periodic directions. Allowed values for

`number`

are 3 for a bulk three-dimensional system, 2 for
a two-dimensional surface slab, and 1 for a one-dimensional system. Default
value is 3.
cellUnit cell parameters in a form of six real values |

`cell ang`

.
content label x y z endContent of the unit cell, where

`label`

is the label of the point charge
Content of the unit cell, where `label`

is the label of the point charge
type and `x y z`

are corresponding Cartesian or fractional crystal coordinates.
Defaults are Cartesian coordinates and atomic units. You can specify Cartesian
coordinates in Å using `content ang`

and fractional coordinates using
`content frac`

. Note that Cartesian coordinates assume that the cell
vector cluster label x y z endAtomic coordinates of the piece of the crystal to be replaced by the QM cluster and surrounding isolation shell (ECPs and explicit point charges), where

`label`

is
the point charge label and `x y z`

are corresponding Cartesian or fractional
crystal coordinates. Defaults are Cartesian coordinates and atomic units. You can
specify Cartesian coordinates in Å using `cluster ang`

and fractional
coordinates using `cluster frac`

.
charges label charge endValues of point charges (for each atom-type) , where

`label`

is the point charge label and
`charge`

specifies charge in atomic units.

ch_list label charge endValues of point charges (for each atom), where

`label`

is the point charge label and
`charge`

specifies charge in atomic units.
Note that `charges`

and `ch_list`

are mutually exclusive.
An integer number *n* can also be
appended to `charges`

or `ch_list`

to set the
tolerance for charge neutrality violation to 10^{-n} (default *n* = 5).

Additionally, the following keywords control the accuracy of PEECM calculation:

`lmaxmom`-

Maximum order of the multipole expansions in periodic fast multipole method (PFMM). Default value is 25. `potval`-

Electrostatic potential at the lattice points resulting from periodic point charges field will be output if this keyword is present. Default is not to output. `wsicl`-

Well-separateness criterion for PFMM. Default is 3.0. `epsilon`-

Minimum accuracy for lattice sums in PFMM. Default is 1.0d-8.