diff --git a/av1_opus_encoder.py b/av1_opus_encoder.py
index 102d5f0..16413e3 100644
--- a/av1_opus_encoder.py
+++ b/av1_opus_encoder.py
@@ -11,7 +11,7 @@ from pathlib import Path
 
 REQUIRED_TOOLS = [
     "ffmpeg", "ffprobe", "mkvmerge", "mkvpropedit",
-    "opusenc", "mediainfo", "av1an", "HandBrakeCLI" # Added HandBrakeCLI
+    "opusenc", "mediainfo", "av1an", "HandBrakeCLI", "ffmsindex" # Added HandBrakeCLI and ffmsindex
 ]
 DIR_COMPLETED = Path("completed")
 DIR_ORIGINAL = Path("original")
@@ -196,11 +196,16 @@ def convert_video(source_file_base, source_file_full, is_vfr, target_cfr_fps_for
         ffmpeg_args += ["-vf", autocrop_filter]
     ffmpeg_args += video_codec_args + [str(ut_video_file)]
     run_cmd(ffmpeg_args)
+
+    print("    - Indexing UTVideo file with ffmsindex for VapourSynth...")
+    ffmsindex_args = ["ffmsindex", "-f", str(ut_video_file)]
+    run_cmd(ffmsindex_args)
+
     ut_video_full_path = os.path.abspath(ut_video_file)
     vpy_script_content = f"""import vapoursynth as vs
 core = vs.core
 core.num_threads = 4
-clip = core.lsmas.LWLibavSource(source=r'''{ut_video_full_path}''')
+clip = core.ffms2.Source(source=r'''{ut_video_full_path}''')
 clip = core.resize.Point(clip, format=vs.YUV420P10, matrix_in_s="709") # type: ignore
 clip.set_output()
 """
@@ -683,6 +688,7 @@ def main(no_downmix=False, autocrop=False, speed=None, quality=None, grain=None)
                     current_dir / f"{file_path.stem}.ut.mkv",
                     current_dir / f"temp-{file_path.stem}.mkv", # This is encoded_video_file
                     current_dir / f"{file_path.stem}.ut.mkv.lwi", 
+                    current_dir / f"{file_path.stem}.ut.mkv.ffindex",
                 ]
                 if handbrake_intermediate_for_cleanup and handbrake_intermediate_for_cleanup.exists():
                     video_temp_files_on_success.append(handbrake_intermediate_for_cleanup)