diff --git a/pkg/obialign/dnamatrix.go b/pkg/obialign/dnamatrix.go
index 9ebf17e..b82b453 100644
--- a/pkg/obialign/dnamatrix.go
+++ b/pkg/obialign/dnamatrix.go
@@ -74,6 +74,30 @@ func _Logaddexp(a, b float64) float64 {
 	return b + math.Log1p(math.Exp(a-b))
 }
 
+func _Log1mexp(a float64) float64 {
+	if a > 0 {
+		log.Panic("Log1mexp: a > 0")
+	}
+
+	if a == 0 {
+		return 0
+	}
+
+	return (math.Log(-math.Expm1(a)))
+}
+
+func _Logdiffexp(a, b float64) float64 {
+	if a < b {
+		log.Panic("Log1mexp: a < b")
+	}
+
+	if a == b {
+		return math.Inf(-1)
+	}
+
+	return a + _Log1mexp(b-a)
+}
+
 // _MatchScoreRatio calculates the match score ratio between two bytes.
 //
 // Parameters:
@@ -83,25 +107,25 @@ func _Logaddexp(a, b float64) float64 {
 // Returns:
 // - float64: the match score ratio when a match is observed
 // - float64: the match score ratio when a mismatch is observed
-func _MatchScoreRatio(a, b byte) (float64, float64) {
+func _MatchScoreRatio(QF, QR byte) (float64, float64) {
 
-	l2 := math.Log(2)
 	l3 := math.Log(3)
+	l4 := math.Log(4)
 	l10 := math.Log(10)
-	lalea := math.Log(4)                   // 1 /(change of the random model)
-	lE1 := -float64(a)/10*l10 - l3         // log proba of sequencing error on A/3
-	lE2 := -float64(b)/10*l10 - l3         // log proba of sequencing error on B/3
-	lO1 := math.Log1p(-math.Exp(lE1 + l3)) // log proba no being an error on A
-	lO2 := math.Log1p(-math.Exp(lE2 + l3)) // log proba no being an error on B
-	lO1O2 := lO1 + lO2
-	lE1E2 := lE1 + lE2
-	lO1E2 := lO1 + lE2
-	lO2E1 := lO2 + lE1
+	qF := -float64(QF) / 10 * l10
+	qR := -float64(QR) / 10 * l10
+	term1 := _Logaddexp(qF, qR)
+	term2 := _Logdiffexp(term1, qF+qR)
 
-	MM := _Logaddexp(lO1O2, lE1E2+l3)                    // Proba match when match observed
-	Mm := _Logaddexp(_Logaddexp(lO1E2, lO2E1), lE1E2+l2) // Proba match when mismatch observed
+	// log.Warnf("MatchScoreRatio: %v, %v , %v, %v", QF, QR, term1, term2)
 
-	return MM + lalea, Mm + lalea
+	match_logp := _Log1mexp(term2 + l3 - l4)
+	match_score := match_logp - _Log1mexp(match_logp)
+
+	mismatch_logp := term2 - l4
+	mismatch_score := mismatch_logp - _Log1mexp(mismatch_logp)
+
+	return match_score, mismatch_score
 }
 
 func _InitNucPartMatch() {
diff --git a/pkg/obikmer/encodefourmer.go b/pkg/obikmer/encodefourmer.go
index c3be879..ba1a5a1 100644
--- a/pkg/obikmer/encodefourmer.go
+++ b/pkg/obikmer/encodefourmer.go
@@ -2,6 +2,7 @@ package obikmer
 
 import (
 	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiseq"
+	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiutils"
 )
 
 var __single_base_code__ = []byte{0,
@@ -131,33 +132,39 @@ func FastShiftFourMer(index [][]int, shifts *map[int]int, lindex int, seq *obise
 	maxshift := 0
 	maxcount := 0
 	maxscore := -1.0
+	maxrelscore := -1.0
 
 	for shift, count := range *shifts {
 		delete((*shifts), shift)
-		score := float64(count)
-		if relscore {
-			over := -shift
-			switch {
-			case shift > 0:
-				over += lindex
-			case shift < 0:
-				over = seq.Len() - over
-			default:
-				over = min(lindex, seq.Len())
-			}
-			score = score / float64(over-3)
+		selectscore := float64(count)
+		relativescore := float64(count)
+		over := -shift
+		switch {
+		case shift > 0:
+			over += lindex
+		case shift < 0:
+			over = seq.Len() - over
+		default:
+			over = min(lindex, seq.Len())
 		}
-		if score > maxscore {
+		relativescore = relativescore / float64(over-3)
+		if relscore {
+			selectscore = relativescore
+		}
+
+		if selectscore > maxscore {
 			maxshift = shift
 			maxcount = count
-			maxscore = score
+			maxscore = selectscore
+			maxrelscore = relativescore
 		} else {
-			if score == maxscore && shift < maxshift {
+			if selectscore == maxscore && obiutils.Abs(shift) < obiutils.Abs(maxshift) {
 				maxshift = shift
 				maxcount = count
+				maxrelscore = relativescore
 			}
 		}
 	}
 
-	return maxshift, maxcount, maxscore
+	return maxshift, maxcount, maxrelscore
 }
diff --git a/pkg/obioptions/version.go b/pkg/obioptions/version.go
index b5d16ab..0d2dd9a 100644
--- a/pkg/obioptions/version.go
+++ b/pkg/obioptions/version.go
@@ -8,7 +8,7 @@ import (
 // corresponds to the last commit, and not the one when the file will be
 // commited
 
-var _Commit = "d3d15ac"
+var _Commit = "4588bf8"
 var _Version = "Release 4.2.0"
 
 // Version returns the version of the obitools package.