Tutorial: Energy evaluations
Qcore provides efficient energy calculations using a variety of standard electronic structure methods, including DFT, GFNxTB, and MP2. Moreover, entos provides quantum embedding approaches, including embedded meanfield theory (EMFT) and ONIOM, to target computational resources at the highaccuracy description of important regions. In this section, we will demonstrate singlepoint energy calculations using DFT, GFNXTB, and EMFT.
1. Density functional theory
The minimal input for running a DFT calculation looks like this:
This input corresponds to a DFT energy calculation on the builtin molecule water using the PBE exchangecorrelation functional and the 631G* AO basis set. All other calculation details such as charge, spin multiplicity, SCF convergence assume default values.
Here is another input file with a greater level of detail:
In this example, we highlight the following options:

The molecular structure can be specified inline using option xyz; alternatively, it can be read from an XYZ file, using the file option within the structure()subcommand.

The total charge of the molecule is specified using the charge option.

The spin multiplicity of the molecule is specified using the multiplicity option.

The maximum number of SCF iterations is specified using the max_iter option.

The print_level option controls the verbosity of output.
Additional options/subcommands for performing DFT calculations can be found in the user manual.
2. GFNxTB
The minimal input for running a GFNxTB calculation looks like this:
As for DFT, the total charge, spin multiplicity and print level can also be specified within the xtb() command.
In cases where the SCF is difficult to converge, we can choose different values for the Fock matrix damping:
3. EMFT
EMFT is a quantumembedding method in which the atoms are partitioned into two subsystems, with the "active" subsystem typically treated at a higher level of meanfield theory and the "environment" subsystem treated at a lower level of meanfield theory. See this paper for a detailed description of the method.
The minimal input for an EMFT energy calculation looks like this:
The EMFT energy calculation is run through the emft() command, which includes the following options/subcommands:

The active subsystem is specified using the active option, whose values are a list of atom indices; the environment subsystem includes the remaining atoms.

The highlevel and lowlevel of theory are specified by the dft()subcommand, each specifies the exchangecorrelation functional and AO basis set for each subsystem.
In the above example, the highlevel subsystem corresponds to the –OH group of the methanol molecule (i.e. atoms 1 and 2 in the molecular structure specification); the highlevel DFT method is B3LYP with Def2TZVP basis set, and the lowlevel DFT method is PBE with Def2SVP basis set.
Examples on specific topics: