Keyword

Options:

Basis | The basis set string Basis defines the global basis set. If absent, the DZVP basis set is used by default. |

The global basis set (DZVP) is used for all atoms not defined explicitly in the BASIS keyword body. In the BASIS keyword body, basis sets can be assigned to individual atoms by a specific atomic symbol (

atomic symbol

element symbol

global basis set

Thus any basis set definition for an atom can be overridden by the explicit assignment of the basis set using the atomic symbol. For example, for CH (see input in 4.1.1), the following basis set definition

Basis (DZVP) C (TZVP) C1 (STO-3G)

assigns an STO-3G basis to atom C1, a TZVP basis to the other carbon and the
DZVP basis to all other atoms. Instead of using basis set strings, the basis set
of an atom may also be specified by the Huzinaga notation given in the `BASIS`
file. In that notation, the foregoing basis set definition would read

Basis (DZVP) C (7111/411/1*) C1 (33/3)

The file `BASIS` contains the basis sets listed in
Table 7. Other basis sets can be obtained from the Extensible
Computational Chemistry Environment Basis Set Database [131] at
www.basissetexchange.org by choosing the deMon2k basis set format.

Modified for Li and Na [155]. |

Instead of reading the basis set from the BASIS file, the user can define the basis directly in the input file according to the format:

Basis SYMBOL Read N L K EXPONENT COEFFICIENT : : EXPONENT COEFFICIENT

Here `SYMBOL` can be an element (e.g. OXYGEN) or atomic symbol
(e.g. O), `N` and `L` are the principal and angular momentum quantum
numbers of this shell and `K`
is the degree of contraction. A shell collects all contracted orbitals
of the same angular momentum quantum number, such as , , or
, , , , and . The contracted
orbitals, , are linear combinations of (atom-centered) Gaussian
type orbitals (LCGTO), which are called the primitive orbitals,
:

(5) | |||

(6) |

The exponents and contraction coefficients are listed in free format under the shell definition line, one line for each primitive orbital (

Multiplicity 3 SCFType ROKS VxcType Basis BLYP Mixing 0.4 PRINT GTO CGTO AUXIS Geometry Z-Matrix O O O r # Variables r 1.207 BASIS O Read 1 0 2 49.98097100 0.4301280000 8.896588000 0.6789140000 2 0 2 1.945237000 0.4947200000E-01 0.493363000 0.9637820000 2 1 2 1.945237000 0.5115410000 0.493363000 0.6128200000 AUXIS (GEN-A2)

In this example the basis set was taken from
www.basissetexchange.org
using the deMon2k format (see Figure 6). The first line
in this format (here 3) must be deleted in the user-defined basis set input.
This line defines the number of contractions and is only needed for the basis
set definition in the file `BASIS`. The basis set in this example contains
a , and shell. Each shell has a contraction degree of two. The
and shells share a common set of exponents. However, in deMon2k both
those shells have to be listed independently, as shown here.

The specification of ECP and MCP valence basis sets will also trigger the use of
the corresponding effective or model core potential if the ECP and MCP keywords
are not specified. Thus, their specification usually is **not necessary**.
An exception is the Hay-Wadt valence basis sets (ECPHW and QECPHW), which
represent alternative choices to the corresponding LANL (Los Alamos National
Laboratory) double valence basis sets. If the Hay-Wadt basis sets are
used, the ECP must be specified explicitly with the ECPS keyword. This is a
special case of the more general capability of defining basis sets and ECPs
or MCPs independently by the BASIS and ECPS or MCPS keywords. **Be aware that
you can mix any ECPs/MCPs with any basis set, including all-electron ones!** Thus,
care must be taken if the keywords BASIS and ECPS/MCPS are used together in the
deMon2k input. For less experienced users, we recommend using the PRINT keyword
with the BASIS options ECPS and MCPS (see example on page
of the tutorial) in order to obtain full information about the basis sets and
ECPs/MCPs actually utilized. If the ECP or MCP valence basis sets are obtained
from external resources like www.basissetexchange.org, the principal
quantum number indexing may be wrong. As a result the tight-binding start density
cannot be generated correctly. As a quick fix, we suggest switching to a CORE start
density by GUESS CORE (see 4.5.5).