|Basis||The basis set string Basis defines the global basis set. If absent, the DZVP basis set is used by default.|
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  at www.basissetexchange.org by choosing the deMon2k basis set format.
Modified for Li and Na .
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,
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).