diff --git a/tools/lammps-gui/lammpsgui.cpp b/tools/lammps-gui/lammpsgui.cpp
index f101006f61..50f9a0b735 100644
--- a/tools/lammps-gui/lammpsgui.cpp
+++ b/tools/lammps-gui/lammpsgui.cpp
@@ -1095,9 +1095,9 @@ void LammpsGui::preferences()
         // must delete LAMMPS instance after preferences have changed that require
         // using different command line flags when creating the LAMMPS instance like
         // suffixes or package commands
+        int newthreads = settings.value("nthreads", 1).toInt();
         if ((oldaccel != settings.value("accelerator", AcceleratorTab::None).toInt()) ||
-            (oldthreads != settings.value("nthreads", 1).toInt()) ||
-            (oldecho != settings.value("echo", 0).toBool()) ||
+            (oldthreads != newthreads) || (oldecho != settings.value("echo", 0).toBool()) ||
             (oldcite != settings.value("cite", 0).toBool())) {
             if (lammps.is_running()) {
                 stop_run();
@@ -1106,6 +1106,10 @@ void LammpsGui::preferences()
             }
             lammps.close();
             lammpsstatus->hide();
+#if defined(_OPENMP)
+            qputenv("OMP_NUM_THREADS", std::to_string(newthreads).c_str());
+            omp_set_num_threads(newthreads);
+#endif
         }
         if (imagewindow) imagewindow->createImage();
     }
diff --git a/tools/lammps-gui/preferences.cpp b/tools/lammps-gui/preferences.cpp
index bdb7d39dbf..91d8026e3f 100644
--- a/tools/lammps-gui/preferences.cpp
+++ b/tools/lammps-gui/preferences.cpp
@@ -39,6 +39,7 @@
 #include <QSpacerItem>
 #include <QTabWidget>
 #include <QVBoxLayout>
+#include <QThread>
 
 #if defined(_OPENMP)
 #include <omp.h>
@@ -115,8 +116,13 @@ void Preferences::accept()
 
     // store number of threads
     QLineEdit *field = tabWidget->findChild<QLineEdit *>("nthreads");
-    if (field)
-        if (field->hasAcceptableInput()) settings->setValue("nthreads", field->text());
+    if (field) {
+        int accel = settings->value("accelerator", AcceleratorTab::None).toInt();
+        if ((accel == AcceleratorTab::None) || (accel == AcceleratorTab::Opt))
+            settings->setValue("nthreads", 1);
+        else if (field->hasAcceptableInput())
+            settings->setValue("nthreads", field->text());
+    }
 
     // store image width, height, zoom, and rendering settings
 
@@ -366,11 +372,11 @@ AcceleratorTab::AcceleratorTab(QSettings *_settings, LammpsWrapper *_lammps, QWi
 
     int maxthreads = 1;
 #if defined(_OPENMP)
-    maxthreads = omp_get_max_threads();
+    maxthreads = QThread::idealThreadCount();
 #endif
     auto *choices      = new QFrame;
     auto *choiceLayout = new QVBoxLayout;
-    auto *ntlabel      = new QLabel("Number of threads:");
+    auto *ntlabel      = new QLabel(QString("Number of threads (max %1):").arg(maxthreads));
     auto *ntchoice     = new QLineEdit(settings->value("nthreads", maxthreads).toString());
     auto *intval       = new QIntValidator(1, maxthreads, this);
     ntchoice->setValidator(intval);