Calculated values of the electronic energy change for the cyclotrimerization of methyl isocyanate^a.

Energy evaluation method	Geometry optimization method
	B3LYP/cc-pVTZ	B3LYP-GD3/cc-pVTZ	MP2/cc-pVTZ
B3LYP/cc-pVTZ	−49.6	−49.6	−50.2
B3LYP-GD3/cc-pVTZ	−58.1	−58.1	−58.6
B3LYP-GD3BJ/cc-pVTZ		−61.7
HF/cc-pVTZ	−56.4	−56.4	−59.4
MP2/cc-pVTZ	−67.3	−67.2	−66.8
CCSD(T)/cc-pVTZ	−70.2
HF/aug-cc-pVTZ	−55.0	−55.1	−58.1
MP2/aug-cc-pVTZ	−68.4	−68.3	−67.8

^aThe energy value is provided in kcal mol⁻¹. The cyclotrimerization enthalpy at 298 K can be calculated by adding the enthalpy correction term to the electronic energy changes provided in the Table. The enthalpy correction terms are +5.1, +5.1, and +5.2 kcal mol⁻¹, respectively, for the B3LYP/cc-pVTZ, B3LYP-GD3/cc-pVTZ, and MP2/cc-pVTZ optimized geometries.