Due to the crucial role played by electron correlation, the accurate determination of ground state geometries of π-conjugated molecules is still a challenge for many quantum chemistry methods. Because of the high parallelism of the algorithms and their explicit treatment of electron correlation effects, Quantum Monte Carlo calculations can offer an accurate and reliable description of the electronic states and of the geometries of such systems, competing with traditional quantum chemistry approaches. Here, we report the structural properties of polyacetylene chains H–(C2H2)N–H up to N = 12 acetylene units, by means of Variational Monte Carlo (VMC) calculations based on the multi-determinant Jastrow Antisymmetrized Geminal Power (JAGP) wave function. This compact ansatz can provide for such systems an accurate description of the dynamical electronic correlation as recently detailed for the 1,3-butadiene molecule [J. Chem. Theory Comput. >2015 11 (2), 508–517]. The calculated Bond Length Alternation (BLA), namely the difference between the single and double carbon bonds, extrapolates, for N →∞, to a value of 0.0910(7) Å, compatible with the experimentaldata. An accurate analysis was able to distinguish between the influenceof the multi-determinantal AGP expansion and of the Jastrow factoron the geometrical properties of the fragments. Our size-extensiveand self-interaction-free results provide new and accurate ab initio references for the structures of the ground stateof polyenes.
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