Improve fallback density estimate

TrimSPlib.js

@@ -3093,24 +3093,28 @@ function rho_fun()
 			}
 		    }
 
-		    // determine the density using a weighted average of the elemental densities
-		    let density_estimate = 0;
+		    // estimate the density from additive elemental molar volumes:
+		    // rho ~= total molar mass / total molar volume
+		    let total_molar_mass = 0;
+		    let total_molar_volume = 0;
 		    for (const key of Object.keys(layer_formula)) {
 		        const amount = Number(layer_formula[key]);
 		        if (!isFinite(amount) || amount <= 0) {
 			    continue;
 			}
-			if (!elemPars[key] || !isFinite(elemPars[key].rho)) {
+			if (!elemPars[key] || !isFinite(elemPars[key].rho) || !isFinite(elemPars[key].A) || elemPars[key].rho <= 0) {
 			    continue;
 			}
-		        if (stoichiometry_sum > 0) {
-			    density_estimate += (amount / stoichiometry_sum) * elemPars[key].rho;
-			}
+			total_molar_mass += amount * elemPars[key].A;
+			total_molar_volume += amount * (elemPars[key].A / elemPars[key].rho);
 		    }
 
-	            rho = density_estimate;
+	            rho = 0;
+		    if (total_molar_volume > 0) {
+			rho = total_molar_mass / total_molar_volume;
+		    }
 		    if (rho > 0) {
-			alert("Warning: Estimated density from elemental constituents; please verify manually.");
+			alert("Warning: Estimated density from additive elemental molar volumes; please verify manually.");
 		    }
 		}
 	    }