#! /bin/sh # A study of the Basis Set Superposition Error (BSSE) in the formation of # Cr(CO)_6. from CO and Cr(CO)_5. # For the BSSE calculation special chemical elements must be created to describe # the 'ghost' atoms, which have zero nuclear charge and mass. They do have basis # functions (and fit functions), however, and they are used to calculate the # lowering of the energy of the system to which the ghost atoms are added, due # to the enlargement of the basis by the ghost basis. The ghost atom has the # same basis and fit set as the normal element but no nuclear charge and no # frozen core. The BASIS key recognizes elements denoted with Gh.atom in the # ATOMS key as being ghost atoms. If the basis file specifies a frozen core ADF # will treat it as if no frozen core is present. # The following calculations are carried out: # 1. CO from C and O. This yields the bond energy of CO with respect to # the (restricted) basic atoms. # 2. CO from the fragments CO (as calculated in 1) and the ghost atom Cr # and 5 Carbon and 5 Oxygen ghost atoms. The ghost atomic fragments # provide basis and fit functions but do not contribute charge or # potential to the molecule. The bond energy of this calculation is # the energy lowering of CO due to the additional basis functions. # This is the BSSE for CO. # 3. Cr(CO)5 from Cr and 5 CO's. This yields the ('normal') bond energy # with respect to the given fragments. # 4. Cr(CO)5 from Cr(CO)5 as fragment (as calculated in 3) but with the # CO basis functions added on the position of the 6th CO ('ghost' CO). # The bond energy is the BSSE for Cr(CO)5 . # 5. Cr(CO)6 with Cr(CO)5 and CO as fragments. The bond energy is the one # without BSSE. This bond energy can now be corrected by the sum of # superposition contributions of calculations 2 and 4. # This series of calculations is carried out with basis set DZ. # Finally, the whole thing might be redone with basis set TZP, to show that the # BSSE decreases with larger basis. # The calculations for the type DZ basis are contained in the sample script # (with input- and output files). Those for type TZP bases can be set up easily # and may be done as an exercise. # For the first series of calculations, with basis type DZ, the input files are # discussed below and the relevant parts are echoed from the output files where # the energy decomposition and the total bond energy are printed. # For the other series, using type TZP basis sets, only a summary of the results # is given. # ===================== # Computational details # ===================== # The calculations in this example all use: # Small core DZ basis set Frozen core level for the Chromium atom: 2p, for # Carbon and Oxygen: 1s Numerical integration precision 4.0 (in Create runs # 10.0, the default) Default settings for model parameters such as density # functional (key XC) and for the remaining computational settings For the BSSE # calculations we first do the 'normal' calculations of CO and Cr(CO)5 , # yielding the fragment files t21.CO and t21.CrCO5. The input files for these # calculations are not shown here. # =========== # BSSE for CO # =========== # For the CO BSSE calculation the standard CO must have been computed first. In # the BSSE run a Cr(CO)5 ghost fragment basis set is then added to the 'normal' # CO input. Important is the use of the BASIS key. In this case the BASIS key is # used for the generation of the ghost atoms, it should have the same definition # for the atoms as will be used later for the Cr(CO)5 fragment. The FRAGMENTS # key is used for the fragment CO. The energy change (the printed 'bond energy' # in the output) is the BSSE. $ADFBIN/adf <