Fix barostat frequency check (#3411)

* fix barostat frequency check

* fix other barostats

* fix error messages

openmmapi/src/MonteCarloAnisotropicBarostat.cpp

@@ -49,8 +49,8 @@ void MonteCarloAnisotropicBarostat::setDefaultPressure(const Vec3& pressure) {
 }
 
 void MonteCarloAnisotropicBarostat::setFrequency(int freq) {
-    if (freq <= 0)
-        throw OpenMMException("Frequency must be positive");
+    if (freq < 0)
+        throw OpenMMException("Frequency cannot be negative");
     frequency = freq;
 }
 

openmmapi/src/MonteCarloBarostat.cpp

@@ -50,8 +50,8 @@ void MonteCarloBarostat::setDefaultPressure(double pressure) {
 }
 
 void MonteCarloBarostat::setFrequency(int freq) {
-    if (freq <= 0)
-        throw OpenMMException("Frequency must be positive");
+    if (freq < 0)
+        throw OpenMMException("Frequency cannot be negative");
     frequency = freq;
 }
 

openmmapi/src/MonteCarloFlexibleBarostat.cpp

@@ -49,8 +49,8 @@ void MonteCarloFlexibleBarostat::setDefaultPressure(double pressure) {
 }
 
 void MonteCarloFlexibleBarostat::setFrequency(int freq) {
-    if (freq <= 0)
-        throw OpenMMException("Frequency must be positive");
+    if (freq < 0)
+        throw OpenMMException("Frequency cannot be negative");
     frequency = freq;
 }
 

openmmapi/src/MonteCarloMembraneBarostat.cpp

@@ -56,8 +56,8 @@ void MonteCarloMembraneBarostat::setDefaultSurfaceTension(double surfaceTension)
 }
 
 void MonteCarloMembraneBarostat::setFrequency(int freq) {
-    if (freq <= 0)
-        throw OpenMMException("Frequency must be positive");
+    if (freq < 0)
+        throw OpenMMException("Frequency cannot be negative");
     frequency = freq;
 }