Electronic-state calculations of diamond cluster passivated by hydrogen bonds Fig.1(a) are performed by complete active space SCF method (CASSCF). The calculation assumes a combination of an self-consistent field (SCF) with a full configuration interaction (CI), involving a subset of the active space with 8 electrons and 8 orbitals i.e. CASCF(8,8).   Next we analyze the C4H12 cluster by describing the defect states as arising from the combination of the atomic orbitals centered on the four nearest  neighbors Fig1(b,c). From fig1(b and c) we see that the four bonds generate a fourfold reducible representation Td symmetry with components A1 Fig.1(b) and T2 Fig.1(c).   Considering that experimentally well established GR1 line at 1.673 eV is due to an 1E - 1T2  transition, we numerically found the 1E- 1T2 excitation energy to be 1.844 ± 0.17eV which is a quite good estimation if we consider that we neglected the influence of  the rest of the infinite lattice.