  The disiloxane molecule is a prime example of silicate compounds containing
the Si-O-Si bridge. The molecule is of significant interest within the field of
quantum chemistry, owing to the difficulty in theoretically predicting its
properties. Herein, the linearisation barrier of disiloxane is investigated
using a fixed-node diffusion Monte Carlo (FNDMC) approach, which is currently
the most reliable {\it ab initio} method in accounting for an electronic
correlation. Calculations utilizing the density functional theory (DFT) and the
coupled cluster method with single and double substitutions, including
noniterative triples (CCSD(T))are carried out alongside FNDMC for comparison.
Two families of basis sets are used to investigate the disiloxane linearisation
barrier - Dunning's correlation-consistent basis sets cc-pV$x$Z ($x = $ D, T,
and Q) and their core-valence correlated counterparts, cc-pCV$x$Z. It is
concluded that FNDMC successfully predicts the disiloxane linearisation barrier
and does not depend on the completeness of the basis sets as much as DFT or
CCSD(T), thus establishing its suitability.
