From d1b2225e92ac3d3c6f7c9316eb7e26bb58c36b1e Mon Sep 17 00:00:00 2001
From: Alberto Hernandez <7albertoh@gmail.com>
Date: Sat, 25 Apr 2026 21:55:33 -0400
Subject: [PATCH] Add qr build option for numerically stable GAP fitting

Add a new meson build option 'qr' (default: true) that enables
the -DHAVE_QR compiler flag. This activates the QR decomposition
solve path in gp_predict.F90, which is numerically stable for
ill-conditioned kernel matrices that cause Cholesky (dpotrf) to
fail with 'LA_Matrix_Factorise: cannot factorise'.

The QR code path already exists in the GAP source (gp_predict.F90,
lines 782-905) but was never wired into the build system.

Fixes issues where gap_fit crashes on SOAP descriptors with
moderate sparse point counts (500-2000).
---
 meson.build   | 8 ++++++--
 meson.options | 1 +
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/meson.build b/meson.build
index b73b35b93..323c841d7 100644
--- a/meson.build
+++ b/meson.build
@@ -27,11 +27,15 @@ add_global_arguments([
 
 if get_option('gap')
   gap_version_res = run_command('src/GAP/gapversion',check:true)
-  add_global_arguments([
+  gap_args = [
     '-DGAP_VERSION='+gap_version_res.stdout().strip(),
     '-DHAVE_GAP',
     '-DHAVE_TB',
-    ], language:'fortran')
+  ]
+  if get_option('qr')
+    gap_args += '-DHAVE_QR'
+  endif
+  add_global_arguments(gap_args, language:'fortran')
 endif
 
 # Determine QUIP_ARCH dynamically based on host machine
diff --git a/meson.options b/meson.options
index 944c33196..1921eb659 100644
--- a/meson.options
+++ b/meson.options
@@ -1,2 +1,3 @@
 option('mpi', type: 'boolean', value : false)
 option('gap', type: 'boolean', value : true)
+option('qr', type: 'boolean', value : true, description : 'Use QR decomposition for numerically stable GAP fitting (requires GAP)')