diff --git a/src/include/ranges/urange.cc.h b/src/include/ranges/urange.cc.h
index 9d5a6a7..40e51ae 100644
--- a/src/include/ranges/urange.cc.h
+++ b/src/include/ranges/urange.cc.h
@@ -363,7 +363,18 @@ namespace CNORXZ
 	template <typename T>
 	static inline Sptr<URange<MetaT>> transform(const RangePtr& r)
 	{
-	    if(r->type() == typeid(PRange<URange<T>>)){
+	    if(r->type() == typeid(CRange)){
+		auto rr = std::dynamic_pointer_cast<CRange>(r);
+		Vector<MetaT> v(rr->size());
+		std::transform(rr->begin(), rr->end(), v.begin(),
+			       [](const T& x) { return static_cast<MetaT>(x); } );
+		return std::dynamic_pointer_cast<URange<MetaT>>
+		    ( URangeFactory<MetaT>(std::move(v)).create() );
+	    }
+	    else if(r->type() == typeid(PRange<CRange>)){
+		return transform<T>( std::dynamic_pointer_cast<PRange<CRange>>(r)->derive() );
+	    }
+	    else if(r->type() == typeid(PRange<URange<T>>)){
 		return transform<T>( std::dynamic_pointer_cast<PRange<URange<T>>>(r)->derive() );
 	    }
 	    else if(r->type() == typeid(URange<T>)){
diff --git a/src/opt/mpi/include/raindex.cc.h b/src/opt/mpi/include/raindex.cc.h
index 6534a60..1696558 100644
--- a/src/opt/mpi/include/raindex.cc.h
+++ b/src/opt/mpi/include/raindex.cc.h
@@ -19,11 +19,16 @@ namespace CNORXZ
 {
     namespace mpi
     {
+	/*===============+
+	 |    RAIndex    |
+	 +===============*/
+	 
 	template <typename T>
 	RAIndex<T>::RAIndex(const T* loc, const RangePtr& range, SizeT lexpos) :
 	    RIndex<YIndex,YIndex>(range, lexpos),
 	    mLoc(loc),
-	    mCur(rank()),
+	    mMin(0),
+	    mMax(0),
 	    mThisRank(getRankNumber())
 	{
 	    setBufferSize();
@@ -35,7 +40,8 @@ namespace CNORXZ
 	RAIndex<T>::RAIndex(const T* loc, const RIndex<YIndex,YIndex>& i) :
 	    RIndex<YIndex,YIndex>(i),
 	    mLoc(loc),
-	    mCur(rank()),
+	    mMin(0),
+	    mMax(0),
 	    mThisRank(getRankNumber())
 	{
 	    setBufferSize();
@@ -68,7 +74,7 @@ namespace CNORXZ
 	template <typename T>
 	const T& RAIndex<T>::operator*() const
 	{
-	    if(mCur != rank()){
+	    if(this->pos() < mMin or this->pos() >= mMax){
 		setBuffer();
 	    }
 	    if(rank() != mThisRank){
@@ -82,7 +88,7 @@ namespace CNORXZ
 	template <typename T>
 	const T* RAIndex<T>::operator->() const
 	{
-	    if(mCur != rank()){
+	    if(this->pos() < mMin or this->pos() >= mMax){
 		setBuffer();
 	    }
 	    if(rank() != mThisRank){
@@ -117,9 +123,51 @@ namespace CNORXZ
 	    std::memcpy(mBuf.data(), d, mBufSize*sizeof(T));
 	    MPI_Bcast(mBuf.data(), mBufSize*sizeof(T), MPI_BYTE, static_cast<int>(rank()),
 		      MPI_COMM_WORLD );
-	    mCur = rank();
+	    mMin = (this->pos() / mBufSize) * mBufSize;
+	    mMax = (this->pos() / mBufSize + 1) * mBufSize;
 	}
-	
+
+	/*===============+
+	 |    RBIndex    |
+	 +===============*/
+
+	template <typename T>
+	RBIndex<T>::RBIndex(T* loc, const RangePtr& range, SizeT lexpos) :
+	    RAIndex<T>(loc, range, lexpos),
+	    mLoc(loc)
+	{}
+
+	template <typename T>
+	RBIndex<T>::RBIndex(T* loc, const RAIndex<T>& i) :
+	    RAIndex<T>(loc, i),
+	    mLoc(loc)
+	{}
+
+	template <typename T>
+	RBIndex<T> RBIndex<T>::operator+(Int n) const
+	{
+	    RBIndex<T> o = *this;
+	    return o += n;
+	}
+
+	template <typename T>
+	RBIndex<T> RBIndex<T>::operator-(Int n) const
+	{
+	    RBIndex<T> o = *this;
+	    return o -= n;
+	}
+
+	template <typename T>
+	RBIndex<T>& RBIndex<T>::set(const T& val)
+	{
+	    if(AI::rank() == AI::mThisRank){
+		mLoc[AI::local()->pos()] = val;
+	    }
+	    AI::setBuffer();
+	    return *this;
+	}
+
+
     } // namespace mpi
 } // namespace CNOXRZ
 
diff --git a/src/opt/mpi/include/raindex.h b/src/opt/mpi/include/raindex.h
index 2ffe939..b5d9f41 100644
--- a/src/opt/mpi/include/raindex.h
+++ b/src/opt/mpi/include/raindex.h
@@ -54,11 +54,32 @@ namespace CNORXZ
 	    
 	    const T* mLoc = nullptr;
 	    mutable Vector<T> mBuf; // used if iterating over content on different rank
-	    mutable SizeT mCur; // current rank in the buffer
+	    mutable SizeT mMin; // current global minimum position in the buffer
+	    mutable SizeT mMax; // current global maximum position in the buffer
 	    SizeT mBufSize;
 	    SizeT mThisRank;
 	};
 
+	template <typename T>
+	class RBIndex : public RAIndex<T>
+	{
+	public:
+	    typedef RAIndex<T> AI;
+	    typedef typename AI::IB IB;
+
+	    DEFAULT_MEMBERS(RBIndex);
+	    RBIndex(T* loc, const RangePtr& range, SizeT lexpos = 0);
+	    RBIndex(T* loc, const RAIndex<T>& i);
+
+	    RBIndex operator+(Int n) const;
+	    RBIndex operator-(Int n) const;
+
+	    RBIndex& set(const T& val);
+	    
+	private:
+	    T* mLoc = nullptr;
+	};
+	
     } // namespace mpi
 } // namespace CNOXRZ
 
diff --git a/src/opt/mpi/include/rarray.cc.h b/src/opt/mpi/include/rarray.cc.h
index 3d84b5a..e94c6e5 100644
--- a/src/opt/mpi/include/rarray.cc.h
+++ b/src/opt/mpi/include/rarray.cc.h
@@ -220,6 +220,12 @@ namespace CNORXZ
 	    return mGeom;
 	}
 
+	template <typename T>
+	const Vector<const T*>& RCArray<T>::buffermap() const
+	{
+	    return mBuf;
+	}
+	
 	template <typename T>
 	template <class TarI, class RTarI, class SrcI, class RSrcI, class I>
 	void RCArray<T>::load(const Sptr<RIndex<TarI,RTarI>>& lpi, const Sptr<RIndex<SrcI,RSrcI>>& ai,
diff --git a/src/opt/mpi/include/rarray.h b/src/opt/mpi/include/rarray.h
index c0414d0..0ec4451 100644
--- a/src/opt/mpi/include/rarray.h
+++ b/src/opt/mpi/include/rarray.h
@@ -115,7 +115,7 @@ namespace CNORXZ
 		loc[r * i] if 'loc' is a purely local array with the same content.
 		The buffer needs to be initialized before according to required range.
 	     */
-	    const Vector<const T*> buffermap() const;
+	    const Vector<const T*>& buffermap() const;
 	    
 	    /** Load all data from other ranks that is accessed by ai in a loop over lpi.
 		imap indicates the global (!) position of ai for a given (local) position of lpi.
diff --git a/src/opt/mpi/tests/CMakeLists.txt b/src/opt/mpi/tests/CMakeLists.txt
index bc7651b..ab81c90 100644
--- a/src/opt/mpi/tests/CMakeLists.txt
+++ b/src/opt/mpi/tests/CMakeLists.txt
@@ -19,3 +19,9 @@ add_dependencies(mpisbutest cnorxz cnorxzmpi test_lib)
 target_link_libraries(mpisbutest ${GTEST_BOTH_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MPI_LIBS} cnorxz cnorxzmpi test_lib)
 set(MPI_TEST_COMMAND mpirun -n 4 mpisbutest)
 add_test(NAME mpisbutest COMMAND ${MPI_TEST_COMMAND})
+
+add_executable(mpiroptest roperation_unit_test.cc)
+add_dependencies(mpiroptest cnorxz cnorxzmpi test_lib)
+target_link_libraries(mpiroptest ${GTEST_BOTH_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT} ${MPI_LIBS} cnorxz cnorxzmpi test_lib)
+set(MPI_TEST_COMMAND mpirun -n 4 mpiroptest)
+add_test(NAME mpiroptest COMMAND ${MPI_TEST_COMMAND})
diff --git a/src/opt/mpi/tests/roperation_unit_test.cc b/src/opt/mpi/tests/roperation_unit_test.cc
new file mode 100644
index 0000000..9611154
--- /dev/null
+++ b/src/opt/mpi/tests/roperation_unit_test.cc
@@ -0,0 +1,166 @@
+// -*- C++ -*-
+/**
+   
+   @file opt/mpi/tests/roperation_unit_test.cc
+   @brief Rank operation unit tests.
+
+   Copyright (c) 2024 Christian Zimmermann. All rights reserved.
+   Mail: chizeta@f3l.de
+   
+**/
+
+#include <cstdlib>
+#include <iostream>
+
+#include "gtest/gtest.h"
+
+#include "cnorxz.h"
+#include "cnorxz_mpi.h"
+#include "test_numbers.h"
+#include "rrange.cc.h"
+#include "rarray.cc.h"
+#include "mpi_env.h"
+
+namespace
+{
+    using namespace CNORXZ;
+    using Test::Numbers;
+    using namespace CNORXZ::mpi;
+
+    class ROp_Test : public ::testing::Test
+    {
+    protected:
+
+	ROp_Test()
+	{
+	    CXZ_ASSERT(getNumRanks() == 4, "exptected 4 ranks");
+	    const SizeT myrank = getRankNumber();
+	    T = 16;
+	    L = 12;
+	    mTRange = CRangeFactory(T).create();
+	    mLRange = CRangeFactory(L).create();
+	    mSpRange = SRangeFactory(Arr<SizeT,4>{0,1,2,3}).create();
+	    mXRange = mTRange*mLRange*mLRange*mLRange;
+	    
+	    RangePtr g1 = CRangeFactory(1).create();
+	    RangePtr g2 = CRangeFactory(2).create();
+	    mGeom = g2*g1*g1*g2*g1;
+	    
+	    mRXRange = rrange(mXRange, g2*g1*g1*g2);
+	    RangePtr scr = mSpRange*mSpRange;
+	    const Vector<Double> vec = Numbers::get(0,mRXRange->sub(1)->size()+2);
+	    RangePtr ltr = mRXRange->sub(1)->sub(0);
+	    RangePtr llr = mRXRange->sub(1)->sub(1);
+	    mMRange = ltr*llr*llr*llr*scr;
+	    Vector<Double> data(mMRange->size());
+	    Vector<Double> data2(mMRange->size());
+	    for(SizeT i = 0; i != mRXRange->sub(1)->size(); ++i){
+		for(SizeT j = 0; j != scr->size(); ++j){
+		    const SizeT k = i*scr->size() + j; 
+		    data[k] = vec[i] + static_cast<Double>(j-scr->size()) / static_cast<Double>((myrank+1)*(T*L*L*L));
+		    data2[k] = vec[i] + static_cast<Double>((j*2-scr->size())*i) / static_cast<Double>((myrank+1)*50);
+		}
+	    }
+	    mM1 = RCArray<Double>( MArray<Double>(mMRange, data), mGeom );
+	    mM2 = RCArray<Double>( MArray<Double>(mMRange, data2), mGeom );
+	    mAll1 = Vector<Double>(data.size() * getNumRanks());
+	    mAll2 = Vector<Double>(data2.size() * getNumRanks());
+	    typedef RIndex<MIndex<CIndex,CIndex,CIndex,CIndex>,MIndex<CIndex,CIndex,CIndex,CIndex>> RI;
+	    const SizeT scrs = scr->size();
+	    for(auto ri = RI(mRXRange); ri.lex() != ri.lmax().val(); ++ri){
+		Double* buf = mAll1.data() + scrs*ri.lex();
+		Double* buf2 = mAll2.data() + scrs*ri.lex();
+		if(ri.rank() == myrank){
+		    std::memcpy(buf, data.data()+ri.local()->lex()*scrs, scrs);
+		    std::memcpy(buf2, data2.data()+ri.local()->lex()*scrs, scrs);
+		}
+		MPI_Bcast(buf, scrs, MPI_DOUBLE, ri.rank(), MPI_COMM_WORLD);
+		MPI_Bcast(buf2, scrs, MPI_DOUBLE, ri.rank(), MPI_COMM_WORLD);
+	    }
+	}
+
+	SizeT T;
+	SizeT L;
+	RangePtr mTRange;
+	RangePtr mLRange;
+	RangePtr mXRange; // global T*L*L*L
+	RangePtr mRXRange;
+	RangePtr mSpRange;
+	RangePtr mMRange;
+	RangePtr mGeom;
+
+	RCArray<Double> mM1;
+	RCArray<Double> mM2;
+	Vector<Double> mAll1;
+	Vector<Double> mAll2;
+    };
+
+    TEST_F(ROp_Test, Check)
+    {
+	EXPECT_EQ(mM1.size(), mM2.size());
+    }
+
+    TEST_F(ROp_Test, Contract)
+    {
+	Vector<Double> comp(mRXRange->size());
+	//RArray<Double> res( MArray<Double>(mMRange), mGeom );
+	typedef UIndex<SizeT> UI;
+	RIndex<MIndex<UI,UI,UI,UI>,MIndex<UI,UI,UI,UI>> x(mRXRange);
+	RIndex<MIndex<UI,UI,UI,UI>,MIndex<UI,UI,UI,UI>> y(mRXRange);
+	SIndex<SizeT,4> A(mSpRange);
+	SIndex<SizeT,4> B(mSpRange);
+	/*
+	// block 1:
+	mM1.load(,A*B*a*b);
+	mM2.load(,A*B*a*b);
+	res(y) += (mM1(x*A*B*a*b) * mM2(xy*B*A*b*a)).c(x*A*B*a*b);
+	// block 2:
+	mM1.load(,A*B*a*b);
+	mM2.load(,A*B*a*b);
+	res(y) += (mM1(x*A*B*a*b) * mM2(xy*B*A*b*a)).c(x*A*B*a*b);
+	// block 3:
+	mM1.load(,A*B*a*b);
+	mM2.load(,A*B*a*b);
+	res(y) += (mM1(x*A*B*a*b) * mM2(xy*B*A*b*a)).c(x*A*B*a*b);
+	// block 4:
+	mM1.load(,A*B*a*b);
+	mM2.load(,A*B*a*b);
+	res(y) += (mM1(x*A*B*a*b) * mM2(xy*B*A*b*a)).c(x*A*B*a*b);
+	*/
+	// comparison loop
+	for(SizeT x0 = 0; x0 != T; ++x0) { VCHECK(x0);
+	for(SizeT x1 = 0; x1 != L; ++x1)
+	for(SizeT x2 = 0; x2 != L; ++x2)
+	for(SizeT x3 = 0; x3 != L; ++x3)
+	for(SizeT y0 = 0; y0 != T; ++y0)
+	for(SizeT y1 = 0; y1 != L; ++y1)
+	for(SizeT y2 = 0; y2 != L; ++y2)
+	for(SizeT y3 = 0; y3 != L; ++y3)
+	for(SizeT A = 0; A != 4; ++A)
+	for(SizeT B = 0; B != 4; ++B) {
+	    const SizeT yi = y0*L*L*L + y1*L*L + y2*L + y3;
+	    const SizeT xi = x0*L*L*L + x1*L*L + x2*L + x3;
+	    const SizeT xy0 = (x0 + y0) % T;
+	    const SizeT xy1 = (x1 + y1) % L;
+	    const SizeT xy2 = (x2 + y2) % L;
+	    const SizeT xy3 = (x3 + y3) % L;
+	    const SizeT xyi = xy0*L*L*L + xy1*L*L + xy2*L + xy3;
+	    const SizeT i1 = xi*4*4 + A*4 + B;
+	    const SizeT i2 = xyi*4*4 + B*4 + A;
+	    comp[yi] += mAll1[i1] * mAll2[i2];
+	}}
+	VCHECK(comp[123]);
+	/*
+	for(auto i = res.begin(); i.lex() != i.lmax().val(); ++i){
+	    EXPECT_EQ(*i, comp[i.lex()]);
+	}
+	*/
+    }
+}
+
+int main(int argc, char** argv)
+{
+    ::testing::InitGoogleTest(&argc, argv);
+    ::testing::AddGlobalTestEnvironment( new MPIEnv(argc, argv) );
+    return RUN_ALL_TESTS();
+}