Cyclopentadiene stability: quantum Monte Carlo, coupled cluster, and density functional theory determinations

The stability of cyclopentadiene is determined using quantum Monte Carlo in both the VMC and DMC variants, HF, CCSD(T), LDA, and the BPW91, B3PW91, BLYP, B3LYP generalized gradient approximation (GGA) density functional theory methods. A comparison with available experimental data shows that the GGAs perform signi®cantly better than LDA and HF for structure optimization. The cyclopentadiene atomization energy calculations reveal that the GGA methods are in fairly good agreement with the DMC result, although a range of 20kcalmol 1 is observed among di!erent functionals. Less satisfactory agreement is obtained with the other methods.