  High-level quantum-chemical calculations are reported at the MP2 and CCSD(T)
levels of theory for the equilibrium structure and the harmonic and anharmonic
force fields of diacetylene, HCCCCH. The calculations were performed employing
Dunning's hierarchy of correlation-consistent basis sets cc-pVXZ, cc-pCVXZ, and
cc-pwCVXZ, as well as the ANO2 basis set of Almloef and Taylor. An empirical
equilibrium structure based on experimental rotational constants for thirteen
isotopic species of diacetylene and computed zero-point vibrational corrections
is determined (r_e^emp: rC-H=1.0615 A, rCtripleC=1.2085 A, rC-C = 1.3727 A) and
in good agreement with the best theoretical structure (CCSD(T)/cc-pCV5Z:
rC-H=1.0617 Angstrom, rCtripleC=1.2083 A, rC-C=1.3737 A). In addition, the
computed fundamental vibrational frequencies are compared with the available
experimental data and found in satisfactory agreement.
