diff --git a/psi4/src/psi4/libsapt_solver/amplitudes.cc b/psi4/src/psi4/libsapt_solver/amplitudes.cc
index db2af063054..a138f410328 100644
--- a/psi4/src/psi4/libsapt_solver/amplitudes.cc
+++ b/psi4/src/psi4/libsapt_solver/amplitudes.cc
@@ -620,7 +620,7 @@ void SAPT2::ijkl_to_ikjl(double *tARAR, int ilength, int jlength, int klength,
 
   for(int i=0; i<ilength; i++) {
     for(int l=0; l<llength; l++) {
-      long int ijkl = i*jlength*klength*llength + l;
+      long int ijkl = (long int)i*jlength*klength*llength + l;
       C_DCOPY(jlength*klength,&(tARAR[ijkl]),llength,X,1);
       for(int j=0; j<jlength; j++) {
         for(int k=0; k<klength; k++) {
diff --git a/psi4/src/psi4/libsapt_solver/disp2ccd.cc b/psi4/src/psi4/libsapt_solver/disp2ccd.cc
index 68627a3a7b0..c46a2e5db8a 100644
--- a/psi4/src/psi4/libsapt_solver/disp2ccd.cc
+++ b/psi4/src/psi4/libsapt_solver/disp2ccd.cc
@@ -240,7 +240,7 @@ void SAPT2p::r_ccd_prep(const char *TARBS, const char *ARBS, const char *CA_RBS,
 
   double **tARBS = block_matrix(occA*virA,occB*virB); //!
 
-  C_DCOPY(occA*virA*occB*virB,&(vARBS[0][0]),1,&(tARBS[0][0]),1);
+  C_DCOPY((size_t)occA*virA*occB*virB,&(vARBS[0][0]),1,&(tARBS[0][0]),1);
 
   double **thARAR = block_matrix(occA*virA,occA*virA); //!
 
@@ -394,7 +394,7 @@ double SAPT2p::r_ccd_energy(const char *TARBS, const char *ARBS, int occA, int v
   psio_->read_entry(PSIF_SAPT_CCD,TARBS,(char *) &(tARBS[0][0]),
     occA*virA*occB*virB*(ULI) sizeof(double));
 
-  double energy = C_DDOT(occA*virA*occB*virB,&(vARBS[0][0]),1,
+  double energy = C_DDOT((size_t)occA*virA*occB*virB,&(vARBS[0][0]),1,
     &(tARBS[0][0]),1);
 
   free_block(vARBS); //!
@@ -419,7 +419,7 @@ double SAPT2p::r_ccd_iterate(const char *TARBS, const char *TARBSerr, const char
   int iter = 1;
   double E_old=0.0, E_new=0.0, RMS=0.0;
 
-  SAPTDIIS diis(PSIF_SAPT_CCD,TARBS,TARBSerr,occA*virA*occB*virB,
+  SAPTDIIS diis(PSIF_SAPT_CCD,TARBS,TARBSerr,(size_t)occA*virA*occB*virB,
     max_ccd_vecs_,psio_);
 
   do {
@@ -569,10 +569,10 @@ double SAPT2p::r_ccd_amplitudes(const char *TARBS, const char *TARBSerr, const c
   psio_->write_entry(PSIF_SAPT_CCD,TARBS,(char *) &(t2ARBS[0][0]),
     occA*virA*occB*virB*(ULI) sizeof(double));
 
-  C_DAXPY(occA*virA*occB*virB,-1.0,tARBS[0],1,t2ARBS[0],1);
+  C_DAXPY((size_t)occA*virA*occB*virB,-1.0,tARBS[0],1,t2ARBS[0],1);
 
-  double RMS = C_DDOT(occA*virA*occB*virB,t2ARBS[0],1,t2ARBS[0],1);
-  RMS /= (double) (occA*virA*occB*virB);
+  double RMS = C_DDOT((size_t)occA*virA*occB*virB,t2ARBS[0],1,t2ARBS[0],1);
+  RMS /= (double) ((size_t)occA*virA*occB*virB);
 
   psio_->write_entry(PSIF_SAPT_CCD,TARBSerr,(char *) &(t2ARBS[0][0]),
     occA*virA*occB*virB*(ULI) sizeof(double));
@@ -712,7 +712,7 @@ double SAPT2p::s_ccd_iterate(const char *SARAR, const char *SARARerr, const char
   int iter = 1;
   double E_old=0.0, E_new=0.0, RMS=0.0;
 
-  SAPTDIIS diis(PSIF_SAPT_CCD,SARAR,SARARerr,occA*virA*occA*virA,
+  SAPTDIIS diis(PSIF_SAPT_CCD,SARAR,SARARerr,(size_t)occA*virA*occA*virA,
     max_ccd_vecs_,psio_);
 
   do {
@@ -1077,9 +1077,9 @@ double SAPT2p::s_ccd_amplitudes(const char *SARAR, const char *SARARerr, const c
   psio_->write_entry(PSIF_SAPT_CCD,SARAR,(char *) &(s2ARAR[0][0]),
     occA*virA*occA*virA*(ULI) sizeof(double));
 
-  C_DAXPY(occA*virA*occA*virA,-1.0,sARAR[0],1,s2ARAR[0],1);
-  double RMS = C_DDOT(occA*virA*occA*virA,s2ARAR[0],1,s2ARAR[0],1);
-  RMS /= (double) (occA*virA*occA*virA);
+  C_DAXPY((size_t)occA*virA*occA*virA,-1.0,sARAR[0],1,s2ARAR[0],1);
+  double RMS = C_DDOT((size_t)occA*virA*occA*virA,s2ARAR[0],1,s2ARAR[0],1);
+  RMS /= (double) ((size_t)occA*virA*occA*virA);
 
   psio_->write_entry(PSIF_SAPT_CCD,SARARerr,(char *) &(s2ARAR[0][0]),
     occA*virA*occA*virA*(ULI) sizeof(double));
@@ -1661,7 +1661,7 @@ void SAPT2p::ccd_iterate(const char *TARAR, const char *TARARerr, const char *Th
   int iter = 1;
   double E_old=0.0, E_new=0.0, RMS=0.0;
 
-  SAPTDIIS diis(PSIF_SAPT_CCD,TARAR,TARARerr,occA*virA*occA*virA,
+  SAPTDIIS diis(PSIF_SAPT_CCD,TARAR,TARARerr,(size_t)occA*virA*occA*virA,
     max_ccd_vecs_,psio_);
 
   do {
@@ -1710,7 +1710,7 @@ double SAPT2p::ccd_energy(const char *TARAR, const char *GARAR, int occA, int vi
   psio_->read_entry(PSIF_SAPT_CCD,TARAR,(char *) &(tARAR[0][0]),
     occA*virA*occA*virA*(ULI) sizeof(double));
 
-  double energy = C_DDOT(occA*virA*occA*virA,&(gARAR[0][0]),1,
+  double energy = C_DDOT((size_t)occA*virA*occA*virA,&(gARAR[0][0]),1,
     &(tARAR[0][0]),1);
 
   free_block(gARAR); //!
@@ -1730,7 +1730,7 @@ double SAPT2p::ccd_amplitudes(const char *TARAR, const char *TARARerr, const cha
   psio_->read_entry(PSIF_SAPT_CCD,ARAR,(char *) &(t2ARAR[0][0]),
     occA*virA*occA*virA*(ULI) sizeof(double));
 
-  C_DSCAL(occA*virA*occA*virA,0.5,&(t2ARAR[0][0]),1);
+  C_DSCAL((size_t)occA*virA*occA*virA,0.5,&(t2ARAR[0][0]),1);
 
   double **gARRA = block_matrix(occA*virA,occA*virA); //!
   double **tARAR = block_matrix(occA*virA,occA*virA); //!
@@ -1993,10 +1993,10 @@ double SAPT2p::ccd_amplitudes(const char *TARAR, const char *TARARerr, const cha
 
   free_block(th2ARAR); //!
 
-  C_DAXPY(occA*virA*occA*virA,-1.0,t2ARAR[0],1,tARAR[0],1);
+  C_DAXPY((size_t)occA*virA*occA*virA,-1.0,t2ARAR[0],1,tARAR[0],1);
 
-  double RMS = C_DDOT(occA*virA*occA*virA,tARAR[0],1,tARAR[0],1);
-  RMS /= (double) (occA*virA*occA*virA);
+  double RMS = C_DDOT((size_t)occA*virA*occA*virA,tARAR[0],1,tARAR[0],1);
+  RMS /= (double) ((size_t)occA*virA*occA*virA);
 
   psio_->write_entry(PSIF_SAPT_CCD,TARARerr,(char *) &(tARAR[0][0]),
     occA*virA*occA*virA*(ULI) sizeof(double));
@@ -2029,7 +2029,7 @@ void SAPT2p::write_IJKL(double **A, int filenum, const char *label, int length_I
   free_block(A);
 }
 
-SAPTDIIS::SAPTDIIS(int ampfile, const char *amplabel, const char *errlabel, int length,
+SAPTDIIS::SAPTDIIS(int ampfile, const char *amplabel, const char *errlabel, size_t length,
                    int maxvec, std::shared_ptr<PSIO> psio) : psio_(psio), vec_label_(amplabel), err_label_(errlabel)
 {
     diis_file_ = 56;
diff --git a/psi4/src/psi4/libsapt_solver/sapt.h b/psi4/src/psi4/libsapt_solver/sapt.h
index 0b5d5126c9c..16e680e6b1c 100644
--- a/psi4/src/psi4/libsapt_solver/sapt.h
+++ b/psi4/src/psi4/libsapt_solver/sapt.h
@@ -64,19 +64,19 @@ class SAPT : public Wavefunction {
   std::shared_ptr<BasisSet> elstbasis_;
   std::shared_ptr<BasisSet> zero_;
 
-  int nsoA_;
-  int nmoA_;
-  int nsoB_;
-  int nmoB_;
-  int ndf_;
-  int noccA_;
-  int foccA_;
-  int aoccA_;
-  int noccB_;
-  int foccB_;
-  int aoccB_;
-  int nvirA_;
-  int nvirB_;
+  size_t nsoA_;
+  size_t nmoA_;
+  size_t nsoB_;
+  size_t nmoB_;
+  size_t ndf_;
+  size_t noccA_;
+  size_t foccA_;
+  size_t aoccA_;
+  size_t noccB_;
+  size_t foccB_;
+  size_t aoccB_;
+  size_t nvirA_;
+  size_t nvirB_;
   int NA_;
   int NB_;
   int natomsA_;
@@ -110,7 +110,7 @@ class SAPT : public Wavefunction {
 
   std::shared_ptr<SAPTDenominator> denom_;
 
-  int nvec_;
+  size_t nvec_;
 
   double **dAR_;
   double **dBS_;
@@ -130,7 +130,7 @@ class CPHFDIIS {
 
 private:
   int max_diis_vecs_;
-  int vec_length_;
+  size_t vec_length_;
 
   int curr_vec_;
   int num_vecs_;
diff --git a/psi4/src/psi4/libsapt_solver/sapt0.h b/psi4/src/psi4/libsapt_solver/sapt0.h
index b846a9eab59..40f38188039 100644
--- a/psi4/src/psi4/libsapt_solver/sapt0.h
+++ b/psi4/src/psi4/libsapt_solver/sapt0.h
@@ -179,11 +179,11 @@ struct SAPTDFInts {
   bool dress_disk_;
   bool active_;
 
-  int i_length_;
-  int j_length_;
-  int ij_length_;
-  int i_start_;
-  int j_start_;
+  size_t i_length_;
+  size_t j_length_;
+  size_t ij_length_;
+  size_t i_start_;
+  size_t j_start_;
 
   double **B_p_;
   double **B_d_;
@@ -206,10 +206,10 @@ struct SAPTDFInts {
 
 struct Iterator {
 
-  int num_blocks;
+  size_t num_blocks;
   int* block_size;
 
-  int curr_block;
+  size_t curr_block;
   long int curr_size;
 
   ~Iterator() { free(block_size); };
diff --git a/psi4/src/psi4/libsapt_solver/sapt2.h b/psi4/src/psi4/libsapt_solver/sapt2.h
index 5fdfa5408f6..ef2cb95201a 100644
--- a/psi4/src/psi4/libsapt_solver/sapt2.h
+++ b/psi4/src/psi4/libsapt_solver/sapt2.h
@@ -38,8 +38,8 @@ class SAPT2 : public SAPT {
   virtual void print_results();
 
 protected:
-  int no_nvirA_;
-  int no_nvirB_;
+  size_t no_nvirA_;
+  size_t no_nvirB_;
 
   double *no_evalsA_;
   double *no_evalsB_;
@@ -47,6 +47,8 @@ class SAPT2 : public SAPT {
   double **no_CA_;
   double **no_CB_;
 
+// These ints should not overflow below 46000 basis functions
+// They contain indices up to nso_ * (nso_ + 1) / 2
   int *ioff_;
   int *index2i_;
   int *index2j_;
diff --git a/psi4/src/psi4/libsapt_solver/sapt2p.h b/psi4/src/psi4/libsapt_solver/sapt2p.h
index 098ca1932af..7e1b4a14a6c 100644
--- a/psi4/src/psi4/libsapt_solver/sapt2p.h
+++ b/psi4/src/psi4/libsapt_solver/sapt2p.h
@@ -171,7 +171,7 @@ class SAPTDIIS {
     int max_diis_vecs_;
 
     int diis_file_;
-    int vec_length_;
+    size_t vec_length_;
 
     int curr_vec_;
     int num_vecs_;
@@ -183,7 +183,7 @@ class SAPTDIIS {
     std::shared_ptr<PSIO> psio_;
 
 public:
-    SAPTDIIS(int, const char *, const char *, int, int, std::shared_ptr<PSIO>);
+    SAPTDIIS(int, const char *, const char *, size_t, int, std::shared_ptr<PSIO>);
     ~SAPTDIIS();
 
     void store_vectors();