Oakes-Renner relation [52]: where the numerical values have been taken from Ref. [20]. The gluon condensate can be extracted from a QSR analysis of charmonium [90], which yields Higher-dimensional condensates involving more quark and gluon fields also exist. For in- stance, the mixed condensate has dimension-five and is given by where M 2 0 = (0 . 8 ± 0 . 1) GeV $^{2}$, which was determined from baryon sum rules [41]. The dimension-six gluon condensate is given by which was also determined in Ref. [90]. Additional condensates include the dimension-six quark condensate and the dimension-eight gluon condensate, which are given in Ref. [89]. In QSR calculations the correlation function (1.97) is evaluated using the OPE (1.99). Contributions that are proportional to the identity operator correspond to purely perturba- tive effects, while contributions from higher dimensional operators in the OPE correspond to non-perturbative effects that are represented through condensates. In practice, the simplest way to calculate these contributions is with the aid of Wick’s theorem (1.16). For instance, consider the calculation of a correlation function of the form where J ( x ) = ¯ q ( x ) q ( x ). The correlation function can be calculated using the perturbative expansion (1.19). The lowest order term in the perturbative expansion involves the following 43 Π $^{(}$Q 2 ) = i$^{∫}$ d $^{4}$x e iq · $^{x}$〈 Ω | T $^{[}$J ( x ) J $^{†}$(0) ] | Ω 〉 , Q 2 = − q $^{2}$, (1.105) 〈 Ω | : g $^{3}$f$_{abc}$ G a αβ (0) G b βγ (0) G c $_{γα}$(0) : | Ω 〉 = 〈 g $^{3}$G$^{3}$〉 = (8 . 2 ± 1 . 0) GeV 2 〈 αG $^{2}$〉 , (1.104) 〈 Ω | : g ¯ q (0) λ a 2 σ $^{µν}$Ga µν (0) q (0) : | Ω 〉 = 〈 ¯ qσGq 〉 = M 2 $_{0}$〈 ¯ qq 〉 , (1.103) 〈 αG $^{2}$〉 = (7 . 5 ± 2 . 0) × 10 − $^{2}$GeV$^{4}$. (1.102) m$_{q}$ 〈 ¯ qq 〉 = − 1 2 f 2 $_{π}$m 2 $_{π}$, f$_{π}$ = 0 . 093 GeV , m$_{π}$ = 0 . 139 GeV , (1.101)