Optimized geometries, energies, vibrational frequencies and normal mode displacement vectors using ab initio SCF calculations at the STO-3G and 3-21G level of theory are reported for two clusters based on the diamond lattice, isotetramantane, C22H28, and methylisotetramantane, C23H30. Additionally, molecular mechanics on larger diamond clusters, approximate to C-140, are also reported. The calculated vibrational spectra, together with the normal mode displacement vectors, are used to assign infrared-visible sum-frequency generation spectra of hydrogen-truncated diamond[111]-(1x1) and methyl-truncated diamond[111]-(1x1).