Excited rotational states in doped 4He clusters: a diffusion Monte Carlo analysis

We report an extension of diffusion Monte Carlo (DMC) to the calculation of the molecular rotational energies by means of the generalized, symmetry-adapted, imaginary-time correlation functions (SAITCFs) originally introduced in the reptation quantum Monte Carlo (RQMC) framework (Škrbić in J Phys Chem A 111:12749, 2007). We studied the a-type and b-type rotational lines of the CO(4He)N clusters with N=1–8 that correlate, in the dimer limit, with the end-over-end and free-rotor transitions. We compare the SAITCF–DMC results with accurate DVR (for the dimer case), RQMC and other DMC data, and with reference experimental findings (Surin in Phys Rev Lett 101:233401, 2008). A good agreement is generally found, but a systematic underestimation of the SAITCF–DMC rotational energies of the b-type series is observed. Sources of inaccuracy in our theoretical approach and in the computational protocol are discussed and analyzed in detail.