Index: flang/lib/Semantics/check-omp-structure.h
===================================================================
--- flang/lib/Semantics/check-omp-structure.h
+++ flang/lib/Semantics/check-omp-structure.h
@@ -229,6 +229,10 @@
   void CheckLoopItrVariableIsInt(const parser::OpenMPLoopConstruct &x);
   void CheckDoWhile(const parser::OpenMPLoopConstruct &x);
   void CheckCycleConstraints(const parser::OpenMPLoopConstruct &x);
+  template <typename T, typename D>
+  bool CheckOperatorValidity(const T &, const D &);
+  template <typename T> void CheckAtomicConstructStructure(T &);
+  void CheckAtomicConstructStructure(const parser::OpenMPAtomicConstruct &);
   void CheckDistLinear(const parser::OpenMPLoopConstruct &x);
   void CheckSIMDNest(const parser::OpenMPConstruct &x);
   void CheckTargetNest(const parser::OpenMPConstruct &x);
Index: flang/lib/Semantics/check-omp-structure.cpp
===================================================================
--- flang/lib/Semantics/check-omp-structure.cpp
+++ flang/lib/Semantics/check-omp-structure.cpp
@@ -1287,11 +1287,108 @@
   }
 }
 
+template <typename T, typename D>
+bool OmpStructureChecker::CheckOperatorValidity(
+    const T &node, const D &variable) {
+  using AllowedBinaryOperators =
+      std::variant<parser::Expr::Add, parser::Expr::Multiply,
+          parser::Expr::Subtract, parser::Expr::Divide, parser::Expr::AND,
+          parser::Expr::OR, parser::Expr::EQV, parser::Expr::NEQV>;
+  using BinaryOperators = std::variant<parser::Expr::Add,
+      parser::Expr::Multiply, parser::Expr::Subtract, parser::Expr::Divide,
+      parser::Expr::AND, parser::Expr::OR, parser::Expr::EQV,
+      parser::Expr::NEQV, parser::Expr::Power, parser::Expr::Concat,
+      parser::Expr::LT, parser::Expr::LE, parser::Expr::EQ, parser::Expr::NE,
+      parser::Expr::GE, parser::Expr::GT>;
+
+  if constexpr (common::HasMember<T, BinaryOperators>) {
+    const auto &variableName{variable.GetSource().ToString()};
+    const auto &exprLeft{std::get<0>(node.t)};
+    const auto &exprRight{std::get<1>(node.t)};
+    if ((exprLeft.value().source.ToString() != variableName) &&
+        (exprRight.value().source.ToString() != variableName)) {
+      context_.Say(variable.GetSource(),
+          "Atomic update variable '%s' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct"_err_en_US,
+          variableName);
+    }
+
+    if constexpr (common::HasMember<T, AllowedBinaryOperators>) {
+      return true;
+    } else {
+      return false;
+    }
+  }
+  return true;
+}
+template <typename T>
+void OmpStructureChecker::CheckAtomicConstructStructure(T &construct) {
+  const auto &dir{std::get<parser::Verbatim>(construct.t)};
+  PushContextAndClauseSets(dir.source, llvm::omp::Directive::OMPD_atomic);
+  const auto &assignment{
+      std::get<parser::Statement<parser::AssignmentStmt>>(construct.t)
+          .statement};
+  const auto &expr{std::get<parser::Expr>(assignment.t)};
+  const auto &var{std::get<parser::Variable>(assignment.t)};
+  std::visit(
+      common::visitors{
+          [&](const common::Indirection<parser::FunctionReference> &x) {
+            const auto &procedureDesignator{
+                std::get<parser::ProcedureDesignator>(x.value().v.t)};
+            const auto &argSpecs{
+                std::get<std::list<parser::ActualArgSpec>>(x.value().v.t)};
+            const parser::Name *name{
+                std::get_if<parser::Name>(&procedureDesignator.u)};
+            if (name &&
+                !(name->source == "max" || name->source == "min" ||
+                    name->source == "iand" || name->source == "ior" ||
+                    name->source == "ieor")) {
+              context_.Say(expr.source,
+                  "Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement"_err_en_US);
+            } else if (name) {
+              bool foundMatch{false};
+              for (const auto &argSpec : argSpecs) {
+                const auto &arg{std::get<parser::ActualArg>(argSpec.t)};
+                if (const auto *parsedExpr{
+                        std::get_if<common::Indirection<parser::Expr>>(
+                            &arg.u)}) {
+                  auto it{parsedExpr->value().source.begin()};
+                  while (it != parsedExpr->value().source.end()) {
+                    std::string iter_val{*it};
+                    if (var.GetSource().ToString() == iter_val) {
+                      foundMatch = true;
+                    }
+                    ++it;
+                  }
+                }
+              }
+              if (!foundMatch) {
+                context_.Say(expr.source,
+                    "Atomic update variable '%s' not found in the argument list of intrinsic procedure"_err_en_US,
+                    var.GetSource().ToString());
+              }
+            }
+          },
+          [&](const auto &x) {
+            if (!CheckOperatorValidity(x, var)) {
+              context_.Say(expr.source,
+                  "Invalid operator in OpenMP ATOMIC construct UPDATE statement"_err_en_US);
+            }
+          },
+      },
+      expr.u);
+}
+
 void OmpStructureChecker::Enter(const parser::OpenMPAtomicConstruct &x) {
   std::visit(
       common::visitors{
-          [&](const auto &someAtomicConstruct) {
-            const auto &dir{std::get<parser::Verbatim>(someAtomicConstruct.t)};
+          [&](const parser::OmpAtomic &atomicConstruct) {
+            CheckAtomicConstructStructure(atomicConstruct);
+          },
+          [&](const parser::OmpAtomicUpdate &atomicConstruct) {
+            CheckAtomicConstructStructure(atomicConstruct);
+          },
+          [&](const auto &atomicConstruct) {
+            const auto &dir{std::get<parser::Verbatim>(atomicConstruct.t)};
             PushContextAndClauseSets(
                 dir.source, llvm::omp::Directive::OMPD_atomic);
           },
Index: flang/test/Semantics/omp-atomic02.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/omp-atomic02.f90
@@ -0,0 +1,109 @@
+! RUN: python %S/test_errors.py %s %flang -fopenmp
+
+! OpenMP Atomic construct
+! section 2.17.7
+! operator is one of +, *, -, /, .AND., .OR., .EQV., or .NEQV
+
+program OmpAtomic
+   use omp_lib
+   CHARACTER c*3, d*3
+   LOGICAL l, m, n
+
+   a = 1
+   b = 2
+   c = 'foo'
+   d = 'bar'
+   m = .TRUE.
+   n = .FALSE.
+   !$omp parallel num_threads(4)
+
+   !$omp atomic
+   a = a + (4*2)
+   !$omp atomic
+   a = a*(b + 1)
+   !$omp atomic
+   a = a - 3
+   !$omp atomic
+   a = a/(b + 1)
+   !$omp atomic
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   a = a**4
+   !$omp atomic
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   c = c//d
+   !$omp atomic
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .LT. b
+   !$omp atomic
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .LE. b
+   !$omp atomic
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .EQ. b
+   !$omp atomic
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .NE. b
+   !$omp atomic
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .GE. b
+   !$omp atomic
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .GT. b
+   !$omp atomic
+   m = m .AND. n
+   !$omp atomic
+   m = m .OR. n
+   !$omp atomic
+   m = m .EQV. n
+   !$omp atomic
+   m = m .NEQV. n
+   !$omp atomic update
+   a = a + (4*2)
+   !$omp atomic update
+   a = a*(b + 1)
+   !$omp atomic update
+   a = a - 3
+   !$omp atomic update
+   a = a/(b + 1)
+   !$omp atomic update
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   a = a**4
+   !$omp atomic update
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   c = c//d
+   !$omp atomic update
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .LT. b
+   !$omp atomic update
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .LE. b
+   !$omp atomic update
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .EQ. b
+   !$omp atomic update
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .GE. b
+   !$omp atomic update
+   !ERROR: Atomic update variable 'l' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   !ERROR: Invalid operator in OpenMP ATOMIC construct UPDATE statement
+   l = a .GT. b
+   !$omp atomic update
+   m = m .AND. n
+   !$omp atomic update
+   m = m .OR. n
+   !$omp atomic update
+   m = m .EQV. n
+   !$omp atomic update
+   m = m .NEQV. n
+   !$omp end parallel
+end program OmpAtomic
Index: flang/test/Semantics/omp-atomic03.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/omp-atomic03.f90
@@ -0,0 +1,164 @@
+! RUN: python %S/test_errors.py %s %flang -fopenmp
+
+! OpenMP Atomic construct
+! section 2.17.7
+! Intrinsic procedure name is one of MAX, MIN, IAND, IOR, or IEOR.
+
+program OmpAtomic
+   use omp_lib
+   real x
+   integer :: y, z
+   x = 5.73
+   y = 3
+   z = 1
+!$omp atomic
+   y = IAND(y, 4)
+!$omp atomic
+   y = IOR(y, 5)
+!$omp atomic
+   y = IEOR(y, 6)
+!$omp atomic
+   y = MAX(y, 7)
+!$omp atomic
+   y = MIN(y, 8)
+
+!$omp atomic
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = IAND(y, 4)
+!$omp atomic
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = IOR(y, 5)
+!$omp atomic
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = IEOR(y, 6)
+!$omp atomic
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = MAX(y, 7)
+!$omp atomic
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = MIN(y, 8)
+
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = MOD(y, 9)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = ABS(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = SQRT(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = SIN(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = COS(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = TAN(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = ASIN(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = ACOS(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = ATAN(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = EXP(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = LOG(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = INT(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = NINT(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = FLOOR(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = FRACTION(x)
+!$omp atomic
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = REAL(x)
+!$omp atomic update
+   y = IAND(y, 4)
+!$omp atomic update
+   y = IOR(y, 5)
+!$omp atomic update
+   y = IEOR(y, 6)
+!$omp atomic update
+   y = MAX(y, 7)
+!$omp atomic update
+   y = MIN(y, 8)
+
+!$omp atomic update
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = IAND(y, 4)
+!$omp atomic update
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = IOR(y, 5)
+!$omp atomic update
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = IEOR(y, 6)
+!$omp atomic update
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = MAX(y, 7)
+!$omp atomic update
+   !ERROR: Atomic update variable 'z' not found in the argument list of intrinsic procedure
+   z = MIN(y, 8)
+
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = MOD(y, 9)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = ABS(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = SQRT(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = SIN(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = COS(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = TAN(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = ASIN(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = ACOS(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = ATAN(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = EXP(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   x = LOG(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = INT(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = NINT(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = FLOOR(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = FRACTION(x)
+!$omp atomic update
+   !ERROR: Invalid intrinsic procedure name in OpenMP ATOMIC construct UPDATE statement
+   y = REAL(x)
+end program OmpAtomic
Index: flang/test/Semantics/omp-atomic04.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/omp-atomic04.f90
@@ -0,0 +1,168 @@
+! RUN: python %S/test_errors.py %s %flang -fopenmp
+
+! OpenMP Atomic construct
+! section 2.17.7
+! Update assignment must be 'var = var op expr' or 'var = expr op var'
+
+program OmpAtomic
+   use omp_lib
+   real x
+   integer y
+   logical m, n, l
+   x = 5.73
+   y = 3
+   m = .TRUE.
+   n = .FALSE.
+!$omp atomic
+   x = x + 1
+!$omp atomic
+   x = 1 + x
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = y + 1
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = 1 + y
+
+!$omp atomic
+   x = x - 1
+!$omp atomic
+   x = 1 - x
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = y - 1
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = 1 - y
+
+!$omp atomic
+   x = x*1
+!$omp atomic
+   x = 1*x
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = y*1
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = 1*y
+
+!$omp atomic
+   x = x/1
+!$omp atomic
+   x = 1/x
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = y/1
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = 1/y
+
+!$omp atomic
+   m = m .AND. n
+!$omp atomic
+   m = n .AND. m
+!$omp atomic
+   !ERROR: Atomic update variable 'm' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   m = n .AND. l
+
+!$omp atomic
+   m = m .OR. n
+!$omp atomic
+   m = n .OR. m
+!$omp atomic
+   !ERROR: Atomic update variable 'm' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   m = n .OR. l
+
+!$omp atomic
+   m = m .EQV. n
+!$omp atomic
+   m = n .EQV. m
+!$omp atomic
+   !ERROR: Atomic update variable 'm' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   m = n .EQV. l
+
+!$omp atomic
+   m = m .NEQV. n
+!$omp atomic
+   m = n .NEQV. m
+!$omp atomic
+   !ERROR: Atomic update variable 'm' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   m = n .NEQV. l
+
+!$omp atomic update
+   x = x + 1
+!$omp atomic update
+   x = 1 + x
+!$omp atomic update
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = y + 1
+!$omp atomic
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = 1 + y
+
+!$omp atomic update
+   x = x - 1
+!$omp atomic update
+   x = 1 - x
+!$omp atomic update
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = y - 1
+!$omp atomic update
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = 1 - y
+
+!$omp atomic update
+   x = x*1
+!$omp atomic update
+   x = 1*x
+!$omp atomic update
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = y*1
+!$omp atomic update
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = 1*y
+
+!$omp atomic update
+   x = x/1
+!$omp atomic update
+   x = 1/x
+!$omp atomic update
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = y/1
+!$omp atomic update
+   !ERROR: Atomic update variable 'x' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   x = 1/y
+
+!$omp atomic update
+   m = m .AND. n
+!$omp atomic update
+   m = n .AND. m
+!$omp atomic update
+   !ERROR: Atomic update variable 'm' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   m = n .AND. l
+
+!$omp atomic update
+   m = m .OR. n
+!$omp atomic update
+   m = n .OR. m
+!$omp atomic update
+   !ERROR: Atomic update variable 'm' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   m = n .OR. l
+
+!$omp atomic update
+   m = m .EQV. n
+!$omp atomic update
+   m = n .EQV. m
+!$omp atomic update
+   !ERROR: Atomic update variable 'm' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   m = n .EQV. l
+
+!$omp atomic update
+   m = m .NEQV. n
+!$omp atomic update
+   m = n .NEQV. m
+!$omp atomic update
+   !ERROR: Atomic update variable 'm' not found in the RHS of the assignment statement in an ATOMIC UPDATE construct
+   m = n .NEQV. l
+
+end program OmpAtomic