diff --git a/R/piping-ini.R b/R/piping-ini.R
index 8d7d1836e..fb1213469 100644
--- a/R/piping-ini.R
+++ b/R/piping-ini.R
@@ -448,7 +448,6 @@
     # This likely cannot be reached because all scenarios should be handled
     # above in the input checking.  The line remains in the code defensively.
     stop("Cannot find parameter '", append, "'", call.=FALSE) # nocov
-
   } else if (appendClean == wLhs) {
     warning("parameter '", lhs, "' set to be moved after itself, no change in order made",
             call. = FALSE)
@@ -608,6 +607,17 @@
 #' @keywords internal
 #' @export
 .iniHandleLine <- function(expr, rxui, envir=parent.frame(), append = NULL) {
+  if (.matchesLangTemplate(expr, str2lang("~diag()"))) {
+    .iniHandleDiag(expr=NULL, rxui=rxui)
+    return(invisible())
+  } else if (length(expr) == 2L &&
+               identical(expr[[1]], quote(`~`)) &&
+               is.call(expr[[2]]) && length(expr[[2]]) >= 2L &&
+               identical(expr[[2]][[1]], quote(`diag`))) {
+    # .matchesLangTemplate(expr, str2lang("~diag(.)")) doesn't work
+    .iniHandleDiag(expr=expr, rxui=rxui)
+    return(invisible())
+  }
   # Convert all variations on fix, fixed, FIX, FIXED; unfix, unfixed, UNFIX,
   # UNFIXED to fix and unfix to simplify all downstream operations
   expr <- .iniSimplifyFixUnfix(expr)
@@ -627,7 +637,6 @@
   } else if (.matchesLangTemplate(expr, str2lang("unfix(.name)"))) {
     expr <- as.call(list(quote(`<-`), expr[[2]], quote(`unfix`)))
   }
-
   if (.matchesLangTemplate(expr, str2lang(".name <- label(.)"))) {
     .iniHandleLabel(expr=expr, rxui=rxui, envir=envir)
   } else if (.matchesLangTemplate(expr, str2lang(".name <- backTransform(.)"))) {
@@ -913,52 +922,24 @@ zeroRe <- function(object, which = c("omega", "sigma"), fix = TRUE) {
   .ret
 }
 
-#' This removes the off-diagonal BSV from a rxode2 model
+#' This removes the off-diagonal BSV from a rxode2 iniDf
 #'
 #' @param ui rxode2 ui model
 #'
 #' @param diag character vector of diagonal values to remove
 #'
-#' @return model with off-diagonals removed
-#'
-#' @export
-#'
+#' @return iniDf with modified diagonal
+#' @noRd
 #' @author Matthew L. Fidler
-#'
-#' @examples
-#'
-#' one.compartment <- function() {
-#'   ini({
-#'     tka <- log(1.57); label("Ka")
-#'     tcl <- log(2.72); label("Cl")
-#'     tv <- log(31.5); label("V")
-#'     eta.ka ~ c(0.6)
-#'     eta.cl ~ c(0.01, 0.3)
-#'     eta.v ~  c(0.01, 0.01, 0.1)
-#'     add.sd <- 0.7
-#'   })
-#'   model({
-#'     ka <- exp(tka + eta.ka)
-#'     cl <- exp(tcl + eta.cl)
-#'     v <- exp(tv + eta.v)
-#'     d/dt(depot) = -ka * depot
-#'     d/dt(center) = ka * depot - cl / v * center
-#'     cp = center / v
-#'     cp ~ add(add.sd)
-#'   })
-#' }
-#'
-.rmDiag <- function(ui, diag=character(0)) {
-  .ui <- rxode2::assertRxUi(ui)
-  .iniDf <- .ui$iniDf
+.iniDfRmDiag <- function(iniDf, diag=character(0)) {
+  .iniDf <- iniDf
   .theta <- .iniDf[!is.na(.iniDf$ntheta),,drop=FALSE]
   .eta <- .iniDf[is.na(.iniDf$ntheta),,drop=FALSE]
   if (length(diag) == 0) {
     .w <- which(.eta$neta1 == .eta$neta2)
     .rmNames <- .eta[-.w, "name"]
-    .eta <- .eta[which(.w),, drop=FALSE]
+    .eta <- .eta[.w,, drop=FALSE]
     .iniDf <- rbind(.theta, .eta)
-    ini(.ui) <- .iniDf
   } else {
     .rmNames <- character(0)
     for (.e in diag) {
@@ -981,13 +962,62 @@ zeroRe <- function(object, which = c("omega", "sigma"), fix = TRUE) {
         .eta <- .eta[.w,,drop=FALSE]
       }
     }
+    .mat <- lotri::as.lotri(.eta)
+    .mat <- lotri::rcm(.mat)
+    class(.mat) <- c("lotriFix", class(.mat))
+    .eta <- as.data.frame(.mat)
+    .eta$err <- NA_character_
     .iniDf <- rbind(.theta, .eta)
-    ini(.ui) <- .iniDf
   }
   if (rxode2.verbose.pipe) {
     for (.v in .rmNames) {
       .minfo(paste0("remove covariance {.code ", .v, "}"))
     }
   }
-  .ui
+  .iniDf
+}
+
+.iniHandleDiag <- function(expr, rxui){
+  if (is.null(expr)) {
+    assign("iniDf", .iniDfRmDiag(rxui$iniDf), envir=rxui)
+  } else {
+    # now get the variables in the diag expression
+    .env <- new.env(parent=emptyenv())
+    .env$names <- character(0)
+    .f <- function(x) {
+      if (is.name(x)) {
+        .env$names <- c(.env$names, as.character(x))
+      } else if (is.call(x)) {
+        lapply(lapply(seq_along(x)[-1], function(i) {x[[i]]}), .f)
+      }
+    }
+    expr <- expr[[2]]
+    lapply(seq_along(expr)[-1],
+           function(i) {
+              .f(expr[[i]])
+           })
+    ## .mat <- as.matrix(lotri::as.lotri(rxui$iniDf))
+    ## attr(.mat, "lotriEst") <- NULL
+    ## class(.mat) <- NULL
+    ## .tmp <- expand.grid(r=.env$names, c=.env$names)
+    ## .tmp <- .tmp[.tmp$r != .tmp$c,]
+    ## .tmp$r <- paste(.tmp$r)
+    ## .tmp$c <- paste(.tmp$c)
+    ## .tmp$r <- vapply(.tmp$r,
+    ##                  function(x) {
+    ##                    which(x == colnames(.mat))
+    ##                  }, integer(1))
+    ## .tmp$c <- vapply(.tmp$c,
+    ##                  function(x) {
+    ##                    which(x == colnames(.mat))
+    ##                  }, integer(1))
+    ## for (.i in seq_along(.tmp$r)) {
+    ##   .mat[.tmp$r[.i], .tmp$c[.i]] <- 0.0
+    ## }
+    ## .mat <- lotri::rcm(.mat)
+    ## class(.mat) <- c("lotriFix", class(.mat))
+    ## .ini <- as.data.frame(.mat)
+    # remove covariates between the expressions
+    assign("iniDf", .iniDfRmDiag(rxui$iniDf, .env$names), envir=rxui)
+  }
 }
diff --git a/R/piping.R b/R/piping.R
index 099c1ee69..e59bcadd9 100644
--- a/R/piping.R
+++ b/R/piping.R
@@ -373,9 +373,17 @@
       # Capture empty arguments (rxode2#688)
       warning("empty argument ignored")
       return(NULL)
+    } else if (is.symbol(.quoted) &&
+                 identical(.quoted, quote(`diag`))) {
+      .quoted <- str2lang("~diag()")
+    } else if (length(.quoted) >= 1 &&
+                 identical(.quoted[[1]], quote(`diag`))) {
+      .quoted <- as.call(c(list(quote(`~`)), .quoted))
     } else if (length(.quoted) == 1) {
       .bracket[i] <- TRUE
       assign(".bracket", .bracket, envir=.env)
+    } else if (identical(.quoted[[1]], quote(`diag`))) {
+
     } else if (identical(.quoted[[1]], quote(`{`)) ||
           identical(.quoted[[1]], quote(`c`)) ||
           identical(.quoted[[1]], quote(`list`))) {
diff --git a/R/ui.R b/R/ui.R
index da481d9cb..d77738e47 100644
--- a/R/ui.R
+++ b/R/ui.R
@@ -137,14 +137,17 @@
 #'
 #' 'omega' values can be set as a single value or as the values of a
 #' lower-triangular matrix.  The values may be set as either a
-#' variance-covariance matrix (the default) or as a correlation matrix for the
-#' off-diagonals with the standard deviations on the diagonals.  Names may be
-#' set on the left side of the \code{~}.  To set a variance-covariance matrix
-#' with variance values of 2 and 3 and a covariance of -2.5 use \code{~c(2, 2.5,
-#' 3)}.  To set the same matrix with names of \code{iivKa} and \code{iivCL}, use
-#' \code{iivKa + iivCL~c(2, 2.5, 3)}.  To set a correlation matrix with standard
-#' deviations on the diagonal, use \code{cor()} like \code{iivKa + iivCL~cor(2,
-#' -0.5, 3)}.
+#' variance-covariance matrix (the default) or as a correlation matrix
+#' for the off-diagonals with the standard deviations on the
+#' diagonals.  Names may be set on the left side of the \code{~}.  To
+#' set a variance-covariance matrix with variance values of 2 and 3
+#' and a covariance of -2.5 use \code{~c(2, 2.5, 3)}.  To set the same
+#' matrix with names of \code{iivKa} and \code{iivCL}, use \code{iivKa
+#' + iivCL~c(2, 2.5, 3)}.  To set a correlation matrix with standard
+#' deviations on the diagonal, use \code{cor()} like \code{iivKa +
+#' iivCL~cor(2, -0.5, 3)}.  As of rxode2 3.0 you can also use
+#' \code{iivKa ~ 2, iivCL ~ c(2.5, 3)} for covariance matrices as
+#' well.
 #'
 #' Values may be fixed (and therefore not estimated) using either the name
 #' \code{fixed} at the end of the assignment or by calling \code{fixed()} as a
@@ -173,6 +176,12 @@
 #' has a label of "Typical Value of Clearance (L/hr)" is \code{tvCL <- 1;
 #' label("Typical Value of Clearance (L/hr)")}.
 #'
+#' Off diagonal values of 'omega' can be set to zero using the
+#' \code{diag()} or \code{diag(iivKa, iivCL)} for example removing all
+#' off-diagonals can be removed with `ini(diag())`, or the off
+#' diagonals between clearance and ka could be removed by
+#' \code{ini(diag(iivKa, iivCL)}.
+#'
 #' \code{rxode2}/\code{nlmixr2} will attempt to determine some
 #' back-transformations for the user.  For example, \code{CL <- exp(tvCL)} will
 #' detect that \code{tvCL} must be back-transformed by \code{exp()} for easier