Using this gauge, the derivative in Eq. (1.107) can be replaced by a covariant derivative. In Ref. [97] the fixed-point gauge expansion of the vacuum expectation value in Eq. (1.107) is explicitly calculated to third order in x . Higher order terms in the expansion can expressed naturally in terms of higher dimensional condensates. For instance, O ( x $^{2}$) terms in the ex- pansion are proportional to the mixed condensate (1.103) while O ( x $^{3}$) terms are proportional to the dimension-six quark condensate. So far we have only considered the leading order term in the perturbative expansion of the correlation function (1.105). Higher order terms that are generated by the perturba- tive expansion (1.19) can be evaluated within the OPE using an approach identical to that described above. However, this naturally leads to time ordered products that include not only the quark fields of the currents in Eq. (1.105), but also quark and gluon fields from the QCD action. This means that vacuum expectation values involving gluon fields will be encountered. Fixed point gauge techniques can be used to express the gluon field in terms of the gluon field strength, and hence a manifestly gauge invariant expansion of vacuum expectation values involving gluon fields can be constructed. Fixed point expansions of vac- uum expectation values involving gluons are discussed in Ref. [97]. Ultimately, the terms in the resulting expansion will lead to contributions from the gluon condensate (1.100) and dimension-six gluon condensate (1.104), for instance. In QSR calculations the OPE is usually truncated at some order and the Wilson coeffi- cients are calculated to a certain order in the coupling α . For instance, Chapter 3 studies heavy quarkonium hybrids which are probed by the current where Q and G αβ a denote a heavy quark field and the gluon field strength, respectively. In Chapter 3 the perturbative, dimension-four 〈 αG $^{2}$〉 and dimension-six 〈 g $^{3}$G$^{3}$〉 gluon condensate contributions are included in the OPE. Because the hybrid current (1.109) contains only heavy quarks, condensates that include light quark fields contribute at higher orders in the expansion and are suppressed. The Wilson coefficients for the perturbative, dimension-four and dimension-six gluon condensate are calculated to leading order in the coupling α . The 45 J$_{µ}$ = g 2 ¯ Qλ $^{a}$γν ˜ G a $_{µν}$Q , ˜ G a µν = 1 2 ϵ$_{µναβ}$G αβ a , (1.109)