RMSEA.MxModel to access the RMSEA of MxModels
RMSEA(x, ci.lower, ci.upper, digits)RMSEA.MxModel,
  RMSEA.summary.mxmodel,
  confint.MxModel,
  extractAIC.MxModel, loadings,
  logLik.MxModel, plot.MxModel,
  residuals.MxModel,
  umxCI_boot, umxCI,
  umxCompare, umxExpCov,
  umxExpMeans, umxFitIndices,
  umxPlotACEcov, umxPlotACE,
  umxPlotCP, umxPlotGxE,
  umxPlotIP,
  umxSummary.MxModel,
  umxSummaryACE, umx_drop_ok,
  umx_standardize_RAM