Standard Model minimal version

The most common low pressure reactor is the Parr hydrogenator shown in Fig. 6.5. This system is composed of a reaction bottle that is coimected to a gas reservoir that has a capacity of about four liters. Two versions of the apparatus are commercially available a standard model that takes 250 mL and 500 mL reaction bottles and that can be used to pressures up to about 60 psig and a larger size that uses 1 liter and 2 liter bottles that have about a 40 psig pressure limit. The bottles are coimected to the gas reservoir by a tube inserted through a rubber stopper in the mouth of the bottle. The common black rubber stoppers may contain catalyst poisons that can be extracted into the reaction mixture by some solvents. The use of neoprene stoppers will minimize this problem. 

This often used benchmark model is represented by the symmetrically stretched H2O molecule at the DZ level, for which the exact FCI results are available (35). In addition to the standard L-CCSD and CCSD correlation energies, we present the results obtained with RMR CCSD and four of its AL versions (Table 5). Similarly as for the S4 model, we employ two different reference spaces the minimal (2,2) space, involving HOMO and LUMO, and the (4,4) space involving four electrons in four orbitals (2 occupied and 2 virtual ones). For an easier comparison, we also present the corresponding differences from the FCI result in parentheses. 

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